Primary Color - Biological Basis

Biological Basis

Primary colors are not a fundamental property of light but are related to the physiological response of the eye to light. Fundamentally, light is a continuous spectrum of the wavelengths that can be detected by the human eye, an infinite-dimensional stimulus space. However, the human eye normally contains only three types of color receptors, called cone cells. Each color receptor responds to different ranges of the color spectrum. Humans and other species with three such types of color receptors are known as trichromats. These species respond to the light stimulus via a three-dimensional sensation, which generally can be modeled as a mixture of three primary colors.

Before the nature of colorimetry and visual physiology were well understood, scientists such as Thomas Young, James Clark Maxwell, and Hermann von Helmholtz expressed various opinions about what should be the three primary colors to describe the three primary color sensations of the eye. Young originally proposed red, green, and violet, and Maxwell changed violet to blue; Helmholtz proposed "a slightly purplish red, a vegetation-green, slightly yellowish (wave-length about 5600 tenth-metres), and an ultramarine-blue (about 4820)". In modern understanding, the human cone cells do not correspond to any real primary colors.

Species with different numbers of receptor cell types would have color vision requiring a different number of primaries. For example, for species known as tetrachromats, with four different color receptors, one would use four primary colors. Since humans can only see to 380 nanometers (violet), but tetrachromats can see into the ultraviolet to about 300 nanometers, this fourth primary color for tetrachromats is located in the shorter-wavelength range.

Many birds and marsupials are tetrachromats, and it has been suggested that some human females are tetrachromats as well, having an extra variant version of the long-wave (L) cone type. The peak response of human color receptors varies, even among individuals with "normal" color vision; in non-human species this polymorphic variation is even greater, and it may well be adaptive. Most placental mammals other than primates have only two types of color receptors and are therefore dichromats; to them, there are only two primary colors.

It would be incorrect to assume that the world "looks tinted" to an animal (or human) with anything other than the human standard of three color receptors. To an animal (or human) born that way, the world would look normal to it, but the animal's ability to detect and discriminate colors would be different from that of a human with normal color vision. If a human and an animal both look at a natural color, they see it as natural; however, if both look at a color reproduced via primary colors, such as on a color television screen, the human may see it as matching the natural color, while the animal does not, since the primary colors have been chosen to suit human capabilities.