Human skin is functionally hairless and exhibits a wide range of natural colors from the most deeply saturated dark brown to pinkish off-white. Differences between people in skin color are readily perceived and have been used as the basis for classifying people into groups referred to as races or race-color identities (Harris et al.,1993). The array of colors observed in the skin of modern humans is greater than that of any other single mammalian species, and is the product of natural selection (Jablonski and Chaplin, 2000), despite some arguments to the contrary (Blum, 1961; Frost, 1988; Robins, 1991; Aoki, 2002). Skin pigmentation in humans evolved primarily to regulate the amount of ultraviolet radiation (UVR) penetrating the skin and, thus, modify its bioactive effects.

Color is imparted to skin by a variety of different substances, which are visible to varying degrees in different people. The most important of these substances is the pigment, melanin, which is produced in specialized cells called melanocytes within the skin. In people with very pale skin, the skin gets most of its color from the bluish-white connective tissue of the dermis and from oxyhemoglobin and deoxyhemoglobin associated with red blood cells circulating in the capillaries of the dermis. The red color produced by circulating hemoglobin becomes more obvious, especially on the face, when the arterioles dilate and become engorged with blood as a result of prolonged exercise or sympathetic nervous stimulation caused by embarrassment or anger (Jablonski, 2006).