The Color perception is, as a part of vision, the ability to perceive light differently depending on the wavelength of electromagnetic radiation. Different spectral compositions of the color stimulus can lead to the same color perception, which is why the composition of the color stimulus cannot be deduced from the perceived color alone. Only if monochromatic light is assumed, light of certain wavelength can also be characterized by the perceived color, whose spectral color is.
History of the exploration
- Isaac Newton discovers that light is composed of different color components and describes the phenomenon of metamerism (differently composed light can produce the same color impression). He coined the phrase "The rays are not colored" (The rays are not colored). Goethe rejected this approach, subjectively seeing "light yellow" and "dark blue" as the basis for his color theory.
- 1794: John Dalton reports about his color vision deficiency. He saw red only as an indistinct shadow, he perceived orange, yellow and green only as different gradations of yellow (this is why the red-green blindness was also called "Daltonism".)
- 1802: Thomas Young suspects that the ability to compose all colors from three primary colors is based on physiological processes in the retina and postulates three types of receptors that match the primary colors.
- James Clerk Maxwell identifies two types of "Daltonism" and explains them with the help of his three-receptor theory.
- Psychologist Ewald Hering developed his four-color theory as a counterthesis. The solution of the contradiction and a further development of the view succeeds von Kries with the Kries zone theory.
- John William Strutt develops the anomaloscope, which is used to test the ability to perceive colors. During examinations he discovers the red and the green faintness.
- In the mid-1960s, two research groups led by Paul K. Brown and Edward McMichael Jr. Microspectrophotometer, which can be used to measure the absorption of individual cones.
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The perceptual system must have at least two different types of "light receptors" to detect different compositions of light. With only one type of receptor, distinctions by wavelength are not possible, which makes W. Rushton 1970  as Principle of univariance highlighted.
Humans possess two different systems of visual receptors. Rods are much more sensitive, but there is only one type of them. Consequently, colors cannot be distinguished with these receptors alone.  The second system is the color receptors, the cones. In humans, there are three types of cones with different spectral sensitivity. They are responsible for day vision (photopic vision). Their stimulus response requires a luminance of at least 0.1 cd/cm². Below this threshold, only light-dark differences are perceivable by the rod receptors (scotopic or night vision). For the perception in the fovea centralis only cones play a role. Under certain twilight conditions, extrafoveal color perception can also be influenced by the rods, but in full daylight these are saturated by the high light intensity and do not contribute to color perception. 
Although in the formation of colors it is necessary to distinguish whether the object perceived as colored emits light or whether it reflects, scatters, diffracts or refracts extraneous light. The incident color stimulus and thus the perception is, however, independent of this.
In everyday life, "color" usually comes from bodies illuminated by light with a continuous light spectrum. Such "white light" is usually emitted by hot bodies of different hues, examples being the sun, the candle flame, or incandescent lamps. Due to developments of newer technology, light sources emitting well defined wavelengths are increasing, caused by electron jumps in the energy levels of atoms. Examples are sodium vapor lamps, LEDs and lasers. Light can be colored by filters, examples are colored glasses of traffic lights. Refraction in media or diffraction at grating structures decomposes light according to wavelengths resulting in different colors, examples are the colors behind a prism or iridescent CDs. Other causes are wave superpositions with interference at thin layers, as in oil lakes. Bodies absorb some wavelengths from the incident "white light", the remitted light is then colored because of the changed spectrum, examples are red blood and green leaves.
Color stimuli of different spectral composition of light can lead to the same color impression (color valence). The red of the traffic light is created by a glass filter that only lets through incandescent light with wavelengths around 650 nm. The red of a glossy beetle or hummingbird can be caused by interference of sunlight, in which certain wavelengths, depending on the thickness of the layer, are amplified while others are extinguished. The different ways of creating the same color impression is called metamerism.
For humans, the electromagnetic radiation of the light spectrum is visible in the wavelength range from 380 to 780 nm. In rare circumstances, spectral vision may extend to short-wave 300 nm or long-wave 820 nm, for example after eye surgery.
Photons can cause a deformation at the proteide of the visual purple in the visual cells (photoreceptors) and trigger electrical signals (receptor potentials) by subsequent biochemical processes. Via the optic nerves, which start in the retina, these signals are transmitted to the central nervous system and processed into a color impression.
There are two systems of photoreceptors in humans.
- The rods are still active at low light intensities below 0.1 cd/cm² and are therefore not sensitive to the Night vision responsible.
- The three different types of Cones register the color valence. Each type of cone has a specific spectral sensitivity.
- L cones (L for long) are sensitive to longer wavelengths. The absorption maximum is about 560 nm, which corresponds to a greenish yellow.
- M cones (M for medium) are sensitive to medium wavelengths. The absorption maximum here is about 530 nm, corresponding to a yellowish green.
- S cones (S for short) are sensitive to shorter wavelengths. The absorption maximum is about 420 nm, a blue. S cones account for only 12 percent of all cones in humans.
Cones do not distinguish wavelengths unambiguously. The absorption spectrum of the cones only indicates the probability with which light of a certain wavelength can trigger an action potential. An action potential may therefore have been triggered by a photon with wavelength A, but also by a photon with wavelength B. To distinguish colors, the brain must compare the responses of at least two different cone types. The more cone types are present, the finer distinctions become possible.
Light-sensitive receptors exist not only in primates such as humans, but also in many different animal species from quite different related groups (vertebrates, arthropods, molluscs).
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- Absorbance is determined here as the number of photons absorbed by a cone per second.
- The absorption maxima mentioned are only approximate values; there are differences not only between species, but also from individual to individual.
Neuronal processing of color stimuli
The first steps of information processing already take place in the retina, which has developed from the eye cup and is embryologically a part of the brain. One group of sensory cells (of cones or rods) converges on each of the retinal ganglion cells via intermediate neurons (bipolar cells), the 3. Neuron. This is referred to as receptive field and distinguish therein a center and its periphery. Photoreceptors of the center act in opposite directions to those of the periphery on the downstream ganglion cell. If the center is excitatory and the periphery is inhibitory, it is called an on-center ganglion cell; if the opposite is true, it is called an off-center ganglion cell. This type of circuitry is used for contrast enhancement.
Essentially, three subsystems are distinguished here:
- Diffuse bipolar cells transmit signals from both L and M cones to so-called parasol-ganglion cells (also called M-cells) further, whose axons are located in the magnocellular layers of the corpus geniculatum laterale (CGL) draw. They show a wide spectral responsiveness. Thus, the information they relay is achromatic and presumably serves primarily to discriminate light from dark.
- The so-called midget-Ganglion cells (also called P cells), on the other hand, receive signals (via midget bipolar cells) from only one L cone or one M cone in the center. The receptive fields of these cells are very small and react differently to long wavelengths and to the light. medium wave light. The axons of the midget ganglion cells move into the parvocellular layers of the CGL. they process mainly the red/green contrast. In evolutionary history this is the youngest subsystem, only in primates the opsins of the L- and M-cones developed by gene duplication.
- On the bistratified-Ganglion cells converge blue-bipolar cells and form an on-center of S cones, diffuse bipolar cells conduct signals from L and M cones, which are inhibitory (Off). This allows especially blue/yellow contrasts to be emphasized. The axons of these retinal ganglion cells project to the coniocellular (sub)layers of the CGL.
In all three cases, horizontal cells participate in the formation of the receptive fields  and amacrine cells modulate the signal flow to the ganglion cells.
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Besides the differentiation of the color qualities, further processing processes are known.
- The fast-working red-green system of the phylogenetically co-evolved M and L cones also serves to emphasize edges in the image pattern. It is the difference of L (red) and M (green) signals compared to the sum of the two. Under laboratory conditions, when both cone types are stimulated with red and green light of equal intensity (isoluminance), the perception of sharp edges decreases sharply (minimally distinct border-Phenomenon).
- The less fast working blue-yellow system is furthermore responsible for color constancy.
- The signal of the red cone alone is presumably used for motion detection especially in slow processes.
Color stimulus, color valence, and color impression
- The color stimulus is the radiant power absorbed in the cones of the retina of the eye. It is the physical cause of color valence and color perception.
- Color valence is the physiological color effect of a radiation. It is characterized by the excitation states of the three cone types of the human eye, which depend on the (physical) color stimulus.
- The perception of color results from the interaction of the (incident) "mean" total brightness and the color constancy powers of the brain. The trichromatic vision – the stimulus response of the three cone types – does not reach consciousness. Along the excitation conduction from the visual cells to the perceiving cerebrum – probably in the corpus geniculatum laterale – the parameter pairs black/white (light value), red/green, blue/yellow (two contrary color pairs) are formed. Evidence for this is also that sensory three-dimensional color space is closer to human color perception than a direct "cone space". The interaction of "bright" and "colorful" can be tested by the Purkinje effect or the Pulfrich effect.
- A spectral color is the narrowly defined, monochromatic color stimulus of a radiation. Monochromatic radiation can come from an emitter (such as a sodium lamp). However, such a color stimulus can also be produced by a monochromator or an interference filter. For every monochromatic radiation in the visible range, there is a certain perception of color. Colors produced by monochromatic radiation are called spectral colors. Monochromaticity of the spectral color is given by the width of the wavelength range – the spectral bandwidth.
Color and brightness
Only above a certain brightness is the triangle formed from three components Color world given, the trichromatic vision with the cones, each containing different opsins (this range of brightness is shown in the V(lambda)-curve). These three cone types, whose excitations provide the color valence of the incident radiation as an inseparable total effect of the three individual excitations, have different Spectral sensitivity curves for the average color-normal observer. Normalized to equal total areas of the three curves we get the Standard spectral value functions. Thus, if each receptor provides 1/3 of the total excitation, then achromatic (white, gray or black) perceived. The magnitude of the total excitation ( $ B+G+R $ ) gives the Color brightness. The hue is given by the relative excitations $ b $ , $ g $ , $ r $: $ b=B/(B+G+R) $ etc. Since $ b+g+r=1 $ , only two components ( $ r $ and $ g $ ) need to be specified to uniquely identify a hue. In an $ r $ – $ g $ plane only one triangle is possible because there are no negative excitations. The corners of the triangle cannot be reached because there is no color stimulus that excites only one color receptor. The spectral color arc does not close. To close the arc, one needs the mixed colors between violet and red, the purple line. In the CIE standard valence system, the standard chromaticity diagram, which is used in DIN 5033.
Theories of color perception
- Three-color theory of Thomas Young (further developed by Hermann von Helmholtz): There are three types of photoreceptors for three colors, which are called primary colors. According to Helmholtz, all other colors including white and black can be produced by additive or subtractive mixing of 3 primary colors, z. B. additive from red-green-blue (RGB).
- Opposite color theory: According to Ewald Hering there is a circle of colors with opposite colors in pairs: red-blue-green, purple-green, blue-orange, violet-yellow. Each pair of opposing colors produces black when mixed subtractively and white when mixed additively.
- Kries zone theory: Johannes von Kries (he worked under Helmholtz) brought both theories together on the basis of neurophysiological research results: At the receptor level, the three-color theory applies, but during processing in the diencephalon, the signals are offset to countercolors.
The cone excitation space as a color space model
Different saturations of the colors towards white or black can be associated with a two-dimensional norm chromaticity diagram do not take into account. For this one needs a three-dimensional structure, the Color space, like for example a sphere, where White pole and a Black pole are present, and a color circle forms the equator.
If all color tones are to be perceived as equidistant from each other, this sphere changes to a strangely shaped Color body. In blue the sphere gets a belly – it becomes more convex. In the case of purple and red, the sphere flattens out, and in the case of yellow, it gets a protruding "knee" – a corner. This subjectively determined color body of perception coincides with the possible color body calculated from the functions of the cone excitations Excitation space.
Metameric color equality
Color stimuli are generated by combinations of different wavelengths of the electromagnetic spectrum. The same color stimulus can be produced by different combinations. This effect is called metamerism. Two color samples can therefore look completely identical, although they absorb different spectral parts of the light. If the color samples are illuminated with colored light – i.e. with light in which spectral components are missing – the difference can become visible, provided that the missing spectral component in one color sample contributes more to its appearance than in the other. This is u.a. a problem in industry when it comes to producing objects from different materials in such a way that they look the same color even under different illumination conditions.
Color vision evolved in adaptation to a changing color quality of illumination, depending on the time of day and season. Long-wave (red) light tends to reach the earth in the morning and evening, short-wave (blue) light tends to reach the earth at midday.
Due to the innate system of color constancy, the object color is perceived as almost unchanged despite different lighting conditions. Without this system, a red cherry would look more white in the morning and more black at noon, an unripe green cherry would look black in the morning and white at noon.
A simple experiment on this can easily be done by looking at a sign 250 (prohibition for vehicles of all kinds) illuminated by the green or red of a traffic light at night and attentively looking at the red ring on the round edge.
In photography, similar conditions can be recreated by shooting with artificial light film during the day or daylight film under artificial light. With a digital camera, such effects can be observed with changes in white balance.
UV perception in humans
Rhodopsin in human rods has two absorption maxima, one in the visible range at 500 nm (turquoise) and a secondary maximum in the UV range at 350 nm. The absorption of UV light in the eye lens normally prevents stimulation in the UV range in the human eye. For the retina, which can be damaged by the high-energy UV radiation, this also represents a protective function. People who have had their lens removed (z. B. because of cataracts) can perceive UV light stimuli as bright, but without doing so as a Color to see. 
defects of color perception
Color vision deficiencies occur in various forms:
- Red-blind people without red receptors are called protanopes (gr. protos, first; gr. at-, not; gr. ope View) denotes
- Green blinds as deuteranopes (gr. deuteros, second), they both show the phenomenon of dichromasia, thus possess only two instead of three cone types.
- Red faintness (protanomaly) and green faintness (deuteranomaly) are based on altered sensitivities of the corresponding receptors.
These visual defects occur with changes in the opsin genes. But also lens discoloration (yellowing) can affect color perception.
As the name suggests, these are nonexistent, unreal colors. In the LMS color space of the cones all perceptible colors can be described. Measurements in preparation of the CIE system and later microspectrophotometric determinations on the eye have been able to objectively determine only the real colors. In principle, any primary valences can be used as coordinates in the three-dimensional color space, resulting in an equally large variety of color spaces. However, this space can be larger than it corresponds to a transformation of the cone space. The "outside" and therefore not perceivable color constructs are called imaginary colors. In order to achieve such color locations metrologically, measurements in color comparison are not changed at the "actual" light, but (factually as subtraction) at the "target" light: external color mixing.
Color perception in the animal kingdom
Color or better "color vision" is a result of nerve activity, a construct of the brain. Animals do not necessarily have a color perception like humans do. "Color in the sense dealt with here" is the perception of different stimuli in light of different wavelengths. The perceptions of the animal species differ considerably in this respect. Causes lie in the history of evolution, in which vision has developed several times and independently of each other. The optical perception apparatuses sometimes have very similar abilities.
There are differences in the number of different receptor types and the stimuli of the wavelengths. In vertebrates, most mammals have two different receptor types, humans and some primates have three, reptiles and the birds that followed from them in evolution often have four color discriminating receptor types.  Many perceptual apparatuses respond to the wavelength range of light that is also visible to humans, and some are also stimulated by light from the ultraviolet or infrared range.
A statement about the subjective color impression of the animals (or other humans) is not possible so far. The reaction to stimuli of different wavelength ranges is experimentally detectable.
Evidence of color perception in the animal kingdom presupposes the ability to learn. It is therefore not entirely clear whether the only weakly developed color vision of the non-state-forming insects, for example, in Drosophila, is a consequence of learning disabilities or a weakness of the visual system.
It is also possible that complex accounting of color information independent of brightness offered no advantage for some nocturnal animals in evolution and therefore did not become established. This would explain why domestic cats, which learn very well and have several cone types, are almost impossible to train to colors: at hunting time at night, gray is more important for the cat.
Even in nocturnal vertebrates, two different cone systems always remain in addition to the rods. The rods for scotopic (night) vision alone could be blinded by daylight, thus the corresponding animal could be nearly blind during the day, in photopic vision. For motion vision in the vertebrate brain, the cones with the longest absorption maximum are evaluated, which leads to an evolutionary advantage when fast movements are to be evaluated. Brightness constancy also requires two perceptual receptors. To enable brightness constancy even under changing illumination conditions, two cone systems are always necessary.
Distribution in the animal kingdom
- In insects, color vision has been studied, especially in the honeybee. Karl von Frisch has shown that bees can be "asked" about their color sensations by training them on colored plates with food rewards. For the proof of real color vision it is not sufficient that an animal returns again and again to the color once experienced as foraging, because it might have learned the gray level. The sensory stimulus Color is only recognized when independent of the brightness is chosen again and again. Frisch tested this by offering the bees color plates of different brightness of the rewarded color in competition with other colors for selection, and found that the color has priority in the decision.
- The mantis shrimp Neogondodactylus oerstedii Has eight different receptor types in the visible and four in the ultraviolet range 
Lower vertebrates and among the mammals the marsupials usually have four cone types, they are therefore called tetrachromats. In addition to the L, M, and S cones, they have an ultraviolet cone that absorbs in the range of less than 380 nm. Since this tetrachromatic color system, which is more complex than in humans, is found in marsupials, birds, and fish, it is thought to represent the original type of vertebrate visual system.
In adaptation to the different lighting conditions of their habitats, the different species of bony fishes have developed different systems. Most of the fishes studied on this so far are tetrachromats. The number of cones and their absorption maxima depends on their way of life: With increasing depth in waters, due to the stronger absorption of long and short wavelength light, the illumination is increasingly monochromatic (monochromatic). In clear seas or lakes, the blue part of the light reaches depths of more than 60 meters. In freshwater lakes with a high density of plankton, yellow-green light prevails at depths of 25 meters; in blackwater rivers and bog lakes, the red component of light reaches a depth of 3 meters at most. At the same time the intensity of light decreases in all waters. Thus, crepuscular fish or fish living in dark regions have predominantly in the red absorbing cones, while diurnal fish living in the upper, light-flooded regions have more blue and green cones.
- Rod monochromats do not have cones, they can see only at very low light intensities and only gray levels. The brightest gray is provided by objects in shades of green.
- Dichromats also have two different cone types, example: common dolphinfish (Coryphaena hippurus).
- Trichromats possess similar to humans three cone types, example: cichlid (Cichlasoma longinasus).
Whether di- and trichromats can also perceive and distinguish different colors depends on further neuronal processing in the retina and brain. 
Chickens possess besides the rhodopsin of the rods four cone pigments for red (absorption maximum at ca. 570 nm), green (ca. 510 nm), blue (ca. 450 nm) and violet (ca. 420 nm). In addition, the pineal organ (pineal gland/epiphysis) contains another pigment, pinopsin (ca. 460 nm). 
Birds and also the reptiles have in their cones oil droplets colored with carotenoids and colorless, which function like a color filter. These filters narrow the absorption spectra of the cone types and thus improve the discriminability of different colors. Mammals, thus also humans, do not possess these color filters.
- Mice have only two cone pigments for green besides the rod pigment rhodopsin (absorption maximum ca. 510 nm) and blue (ca. 350 nm). That dogs do not have a color sense, i.e. they see black and white, is still claimed even in popular scientific articles  . But even the domestic dog has two cone types with sensitivities in the green and blue spectral range.
- Primates can see "in color". As studies on monkeys at the Japanese National Research Institute in Tsukuba have shown , the ability to perceive hues independently of brightness is not innate. This was observed in monkeys that grew up in monochromatic light. They could not recognize a colored object whenever it reflected light of different wavelengths under different illumination conditions.
Many insects, birds, lizards, turtles and fish have receptors in their retina that are also stimulated by light shorter than 400 nm, i.e. by ultraviolet light. 
- Sir John Lubbock, a friend and neighbor of Charles Darwin, noted before 1882 that ants pick up their pupae under ultraviolet (UV) and carry them out of the radiation range.
- In the 1950s, Karl von Frisch found that bees and ants perceive UV light as color.
Due to the fourth type of cone, which has its absorption maximum in ultraviolet (UV), tetrachromatic animals such as some insects, almost all fishes (goldfish), reptiles, the primitive mammals of Australia and birds can distinguish more colors than man. Studies on the budgerigar (Melapsittacus undulatus) showed that the bird can perceive not only the colors that humans also distinguish, but also mixtures with different UV content. For example, depending on the UV component, a bird distinguishes different colors at a certain blue, where humans can only perceive a single one.
However, from the number of cone types it cannot be directly concluded that animals can also distinguish the corresponding number of colors. This depends on the further processing of color information in the retina and brain and can only be investigated by behavioral experiments.
- The ability to perceive ultraviolet plays a role in courtship for some birds.
- Measurements of UV reflectance showed that of 139 species in which males and females cannot be distinguished by the human eye, in more than 90% of species the sexes differ in the UV pattern. 
- In males of 108 Australian bird species, those areas of the plumage that play a role in courtship reflect more UV than other plumage areas. 
- In the blue tit (Parus caeruleus) the females preferentially choose the males that reflect the most UV. Since the reflection of UV depends on the microstructure of the feathers, it can provide information about the health of the males.
- In the case of the azure bishop (Guiraca caerulea), the males with the highest UV reflectance occupy the largest and most productive territories and feed their young most frequently. 
But also the perception of ultraviolet or its effects plays a role in the acquisition of food.
- The surface of many fruits reflects ultraviolet light. This makes it easier for animals with the ability to perceive ultraviolet to find them. 
- Kestrels (Falco tinnunculus) detect the track of their prey (voleMicrotus agrestis) based on their markings, as urine and feces reflect ultraviolet. 
Unlike other plants or the leaves, ripe yellow bananas fluoresce blue in the ultraviolet. This could be a clue that banana-eating animals can detect maturity this way. 
Genetics of color vision
In humans, males are more likely to be red-green blind than females because the gene for the corresponding visual pigment is encoded on the X chromosome, of which females have two but males only one. Therefore, if there is a gene defect on one X chromosome, usually a functional gene product can still be produced in females because there is still an intact gene on the other X chromosome, but not in males.
Evolution of the cone types
Birds have four cone types, whose absorption maxima are at 370 nm (UV type), 445 nm (S type), 508 nm (M type), and 565 nm (L type). Based on comparisons of DNA sequences of different opsin types in different recent animals, it is thought that the common ancestors of birds and mammals also possessed four cone types. At an early stage of mammalian evolution, the middle S and M types were lost. It is assumed that these animals were nocturnal and therefore could tolerate this change in the visual system. About 40 million years ago, with the transition to daytime activity in the ancestors of Old World primates, a third type of cone arose by gene duplication, so that an M-type (530 nm) was again available, but its maximum absorption differed only slightly from the L-type (560 nm). A selective advantage may have been that three cone types were better at distinguishing fruit as a food source than two. 
Although the neuronal pathways and mechanisms of processing color information in humans are known in principle, how the brain "translates" the activity of neurons into a mental image, i.e. ultimately the process of becoming aware of color, is not known. "Apparently, it is not the primary developmental task of the sense of sight to produce aesthetic sensations. Rather, its most important task seems to be to ensure the survival of the individual by safe orientation and by optimal recognition. Therefore, the sense of sight has evolved to adapt to lighting conditions as much as possible." ( Harald Kuppers  ) Indications that at this stage of perception there are cultural differences, and thus differences influenced by learning processes, are given by the naming of colors and the division of the color spectrum into color groups.
Color names and color system
- Empedocles understands white and black as colors.
- Aristotle puts in his work De sensu ("On the senses") the brightness of the air equals the color white of bodies, darkness equals the color black. The colors are composed of different mixtures of white and black.
According to these ideas, the colors have been used until the 17. In the twentieth century, the colors are arranged according to a brightness scale: White – Yellow – Red – Blue – Black. While nowadays a color is defined by hue, saturation and brightness, until then hue was seen only as a consequence of brightness. 
This point of view is also reflected in the etymology of the color names: Thus, the terms white and yellow go back to a common Indo-Germanic linguistic root with the meaning ‘bright, shiny, shining’ (fr: blanc, it: bianco =white) can be traced back.
Seeing and hearing
Visually the variety of the world is taken up by a "receptor surface. The peculiarity of color vision is that in a narrowly limited area of the retina, through the cones of the Color stimulus is recorded. In the sense of hearing, each frequency is perceived by its own receptor at two (opposite) locations. The spectral, i.e. visual, diversity of the outside world is mapped onto three stimulus variables. Other creatures have other visual systems, but humans perceive metameric stimuli as the same, thus a color recreation, the recreation of a color by other conditions, is possible at all. Musical instruments, on the other hand, can be clearly distinguished on the basis of overtones, for example.
The illusion of a colored world
As soon as one sets the consciousness of man in contrast to its material basis, i.e. the brain and the objective environment, color becomes an independent object. As such "object color" it does not exist in the environment.
"Colors are brain-generated qualities of experience of mere electromagnetic radiation in an absolutely colorless world."
– Eckart Voland  
"Rays are not colored"
"(Light) rays are not colored"
– Newton 
The brain researcher Gerhard Roth argues that the experienced ‘reality’ of our world (incl. Colors and music) is only the reality interpreted by our brain. 
The philosophy of mind and the neurosciences are dedicated to the qualitative character of color perception.
From the point of view of physics, there are electromagnetic waves of different wavelengths and thus of different energy content. This color stimulus evokes interactions in different sensory cells. In this way, a different value of the depth of reaction, the color valence, is created causally in the sensory cells and transmitted in the central nervous system. color becomes perceptible, an objectively ascertainable condition. In a long development a system for the visual observation of the environment has arisen. The perception of colors allows orientation and movement in the world. Dangers or amenities can be recognized. The environment transmits information in the rays of light. Color is a translation of this information in the environment by the nervous system.
In European culture, the many different shades of color are assigned to a few color categories: Violet, Blue, Green, Yellow, Orange, Red, Pink, Brown. Investigations showed that the Berimos on Papua New Guinea used only five categories (s. also prototype semantics). Thus, they assign a wide range of color shades, which Europeans divide into the two categories green and blue, to only one term.