Many factors governe skin color such as thickness and consisteny of the stratum corneum, epidermis and connective tissue, blood supply, blood color, presence of caratene, and melanin. Melanin is the most important factor.  The epidermis and dermis contahn melanocytes- cells capable of melanin synthesis. The melanocytes originate in the neural crest. The melanin granules in the melanocytes differ the light-skinned races from the dark-skinned races. But, the number of melanocytes don't differ them. There as as many melanocytes in black skin as in white skin. The amhno acid tyrosine, the copper-bearing enzyme tyrosine and oxygen affect pigment formation in the melanocytes.  Tyrosinase oxidizes tyrosine via dihydroxyphenylalanine (dopa), dopaquinone, and other intermediate states to melanin. While the first oxidation stage requires the presence of an active enzyme-tyrosinase, the further steps are instigated by other more or less nonspecific factors. Non specific factor such as radian heat, can change precursors of melanin.

Increased pigmentation in liver disease is attributed to increase amounts of tyrosine in the blood.  Pigmentation is reduced by the absense of tyrosine in phenylketonuria.  Increased pigmentation is induced by ACTH. Hydrocortisone and pharmacologically related substances inhibit the discharge of melanocyte-stimulant hormone and ACTH and consequently pigmentation. Addison's disease- inactivation of the adrenal cortex- cause increase hormone release.

Oxidation beyond the normal extent converts melanin to a colorless form, a proces that is exploited cosmetically in bleaching hair. Long wave UV (UV-A) cause pigmentation minutes after exposure in most subjects and lasts for week. Short-wave UV(UV-B) initiallz causes erythema ; tanning follows one day later, reaches its maximum only after several days, but fades only slowly over months. (HC -Team)