Trichromatic vision

Trichromatic color vision is the ability of humans and some other animals to see different colors, mediated by interactions among three types of color-sensing cone cells. Each of the three types of cone in the retina of the eye contains a different type of photosensitive pigment, which is composed of a transmembrane protein called opsin and a light-sensitive molecule called 11-cis retinal. Each different pigment is especially sensitive to a certain wavelength of light (that is, the pigment is most likely to produce a cellular response when it is hit by a photon with the specific wavelength to which that pigment is most sensitive). The three types of cones are L, M, and S, which have pigments that respond best to light of long (especially 560 nm), medium (530 nm), and short (420 nm) wavelengths respectively (Kandel et al., 2000).

Since the likelihood of response of a given cone varies not only with the wavelength of the light that hits it but also with its intensity, the brain would not be able to discriminate different colors if it had input from only one type of cone. Thus, interactions between at least two types of cone is necessary to produce the ability to perceive color. With at least two types of cones, the brain can compare the signals from each type and determine both the intensity and color of the light. For example, moderate stimulation of a medium-wavelength cone cell could mean that it is being stimulated by very bright red (long-wavelength) light, or by not very intense yellowish-green light. But very bright red light would produce a stronger response from L cones than from M cells, while not very intense yellowish light would produce a stronger response from M cells than from other cells (counterintuitively, a "strong response" here refers to a large hyperpolarization, since rods and cones communicate that they are being stimulated by not firing). Thus trichromatic color vision is accomplished by using combinations of cell responses.

Reference

 * Kandel E.R., Schwartz, J.H., Jessell, T.M., Principles of Neural Science, 4th ed., pp.577-580. McGraw-Hill, New York (2000).