In plants the major pigments are the carotenes (reddish orange to yellow), the anthocyanins (red, blue, and violet), and the chlorophylls (green). The red and yellow colors of autumn foliage are due to the exposure of the anthocyanins after the green chlorophyll pigments, which usually mask them, have decomposed and faded. The major animal pigments are the hemes (red) of blood hemoglobin, the carotenes, the melanins (black and brown), and guanine (white and iridescent). The latter three produce the surface coloration of most animals.
Pigments not only provide external coloration but also function in some important physiological processes. In the retina of the eye the pigment cells (rods and cones) adjust or regulate the entering light (see vision). Among its other functions, carotene operates in the synthesis of vitamins and of chlorophyll. Chlorophyll is essential for plant photosynthesis. Hemoglobin in the blood carries oxygen for respiration. Chlorophyll and hemoglobin are structurally quite similar, both belonging to the pyrrole group of pigments.
In humans the degree of darkness of the skin, hair, and iris of the eye depends primarily on the amount of melanin present. The presence of hemoglobin and carotene in the blood contributes to skin color. Moles and freckles are caused by high local concentrations of melanin; albinism by a lack of melanin; and some birthmarks, e.g., "strawberry marks," by an unusual local proliferation of blood vessels (and hence of hemoglobin) near the skin surface. Tanning of human skin results from an increase of melanin production under the stimulation of ultraviolet light.
The coloration of an organism may be caused by deposits of organic pigments in the tissues (as in human skin or in plant leaves), by optical effects of the refraction of light rays (as in mollusk shells and in some butterfly wings and bird feathers), or by a combination of both (see color). The different modes are illustrated in the baboon and the mandrill: the predominantly brown coloring is due to melanin, but the red and blue markings are also caused by melanin, in the latter case by the refraction of light due to specific spatial arrangements of the pigment granules in the skin areas involved.
The pigmentation of many animals is adapted to their environment and aids in their survival (see mimicry; protective coloration). In some animals the pigment is changeable; the flounder and the squid, for example, are capable of adapting themselves to the color of their background and thus often of escaping detection by their enemies. The exact mechanism of such changeability is not clearly understood, but in most cases it is due primarily to visual stimulation. In the squid the chromatophores (containing melanin granules) are controlled by muscles and can expand from an almost invisible pinpoint to 60 times their original size, giving the whole animal a dark appearance. Pigmentation changes are also at least partially controlled by hormones—as, in part, is pigmentation synthesis itself.