One of the main tasks of vision is to distinguish objects from their backgrounds, which at a minimum requires detection of spatial differences in patterns of retinal illumination. This capability is greatly enhanced if the organism can also discern differences in the wavelengths of the light forming the retinal image. These spectral or chromatic patterns can also be useful in the identification, as well as the detection, of important objects such as food sources or predators. The spectral composition of the retinal image of a given object depends on the spectral content of the incident light, the object's reflectance characteristics, and to some extent the differential absorption of certain wavelengths in the ocular media before light reaches the retinal photoreceptors. It is the photoreceptors that first dissect the retinal image into its chromatic components, and in the vertebrate retina this process depends on the cones.

Color vision generally requires the presence in the retina of two or more photopigments with different spectral sensitivities. As described previously, the photopigments of all animals are composed of an opsin, a large, membrane-spanning protein, and the chromophore, 11-cis retinaldehyde. When the chromophore absorbs a light quantum, it changes shape and activates the opsin, which then functions as a catalyst for further reactions in the photoreceptor (see Chapter 5). Retinaldehyde by itself is colorless, but when combined with the opsin its geometry is modified, and the combination becomes a photopigment that absorbs light in the visible range. Changes in one amino acid group at critical points in the opsin can significantly alter the spectral sensitivity of the opsin-chromophore combination.