Skin has color. It may come from the materials that make the skin, from pigment(s) within the skin, or from the color of fluids flowing through the skin.
Without pigment or blood, the skin has a yellow to gray or translucent color, in the Caucasian with clear skin, “high” color in the cheeks is given by blood flow through the capillaries which can open or close according to circumstances: temperature; physical effort; emotion, etc. If the person suffers from cardiac failure or one of many pulmonary problems, the skin may be tinged blue (cyanosis) by the deoxygenated blood flowing in the capillaries.
Most persons have some degree of pigmentation of their skin. This is produced by melanocytes (formerly called melanoblasts) located in the deepest layer of the epidermis, immediately superficial to the dermis. These cells have long pigment-bearing processes which extend superficially between the other cells of the epidermal layer; the pigment then seems to be liberated and to move upwards to the surface as a fine “dust” into the outermost horny layer, stratum corneum.
These cells are under the control of the adrenal melanocyte stimulating hormone (MSH) itself under the control of adrenocorticotrophic hormone (ACTH), which explains hyperpigmentation found in Addison’s disease, an abnormality of adrenal cortex function. From tyrosine, catalyzed by tyrosinase at least two forms of melanin are produced. Of these, the black pigment, eumelanin, is most widespread through the skin and hair, and pheomelanin, a red pigment, is less widely distributed in red hair, the lips, vagina, glans of the penis and nipple of the breast.
The amount of melanin in the skin, and the character of the melanin, is controlled by the genes we inherit. It is accepted that melanin blocks ultraviolet light, and that increased deposition follows exposure to UV light (tanning), but most scientists abhor the teleological argument which postulates that Africans are dark because they live on the equator and need protection from the sun. Were that so one would have to explain why West Africans are more favored than East Africans, and the population of Ecuador living on the equator is hardly protected at all!
Although not apparently related to melanin, and possibly a factor of genetic influence concomitant with the gene that controls melanin formation, the surgeon knows persons with dark skin of African antecedence are 15 times more likely to develop excess thickening of their scars (keloid formation) than are those of Caucasian heritage with poorly pigmented skin.