The ability of the human eye to distinguish colours is based upon the varying sensitivity of different cells in the retina to light of different wavelengths. Humans are trichromatic—the retina contains three types of colour receptor cells, or cones. One type, relatively distinct from the other two, is most responsive to light that is perceived as blue or blue-violet, with wavelengths around 450 nm; cones of this type are sometimes called short-wavelength cones, S cones, or blue cones. The other two types are closely related genetically and chemically: middle-wavelength cones, M cones, or green cones are most sensitive to light perceived as green, with wavelengths around 540 nm, while the long-wavelength cones, L cones, or red cones, are most sensitive to light is perceived as greenish yellow, with wavelengths around 570 nm.
Light, no matter how complex its composition of wavelengths, is reduced to three colour components by the eye. Each cone type adheres to the principle of univariance, which is that each cone’s output is determined by the amount of light that falls on it over all wavelengths. For each location in the visual field, the three types of cones yield three signals based on the extent to which each is stimulated. These amounts of stimulation are sometimes called tristimulus values.
The response curve as a function of wavelength varies for each type of cone. Because the curves overlap, some tristimulus values do not occur for any incoming light combination. For example, it is not possible to stimulate only the mid-wavelength (so-called “green”) cones; the other cones will inevitably be stimulated to some degree at the same time. The set of all possible tristimulus values determines the human colour space. It has been estimated that humans can distinguish roughly 10 million different colours.
The other type of light-sensitive cell in the eye, the rod, has a different response curve. In normal situations, when light is bright enough to strongly stimulate the cones, rods play virtually no role in vision at all. On the other hand, in dim light, the cones are under stimulated leaving only the signal from the rods, resulting in a colourless response. (Furthermore, the rods are barely sensitive to light in the “red” range.) In certain conditions of intermediate illumination, the rod response and a weak cone response can together result in colour discriminations not accounted for by cone responses alone. These effects, combined, are summarized also in the Kruithof curve, that describes the change of colour perception and pleasantness of light as function of temperature and intensity.
While the mechanisms of colour vision at the level of the retina are well-described in terms of tristimulus values, colour processing after that point is organized differently. A dominant theory of colour vision proposes that colour information is transmitted out of the eye by three opponent processes, or opponent channels, each constructed from the raw output of the cones: a red–green channel, a blue–yellow channel, and a black–white “luminance” channel. This theory has been supported by neurobiology, and accounts for the structure of our subjective colour experience. Specifically, it explains why humans cannot perceive a “reddish green” or “yellowish blue”, and it predicts the colour wheel: it is the collection of colours for which at least one of the two colour channels measures a value at one of its extremes.
The exact nature of colour perception beyond the processing already described, and indeed the status of colour as a feature of the perceived world or rather as a feature of our perception of the world – a type of qualia – is a matter of complex and continuing philosophical dispute.
When an artist uses a limited colour palette, the eye tends to compensate by seeing any grey or neutral colour as the colour which is missing from the colour wheel. For example, in a limited palette consisting of red, yellow, black, and white, a mixture of yellow and black will appear as a variety of green, a mixture of red and black will appear as a variety of purple, and pure grey will appear bluish.
The trichromatic theory is strictly true when the visual system is in a fixed state of adaptation. In reality, the visual system is constantly adapting to changes in the environment and compares the various colours in a scene to reduce the effects of the illumination. If a scene is illuminated with one light, and then with another, as long as the difference between the light sources stays within a reasonable range, the colours in the scene appear relatively constant to us. This was studied by Edwin Land in the 1970s and led to his retinex theory of colour constancy.