How does melanin concentration affect human skin color?

Human skin color is primarily determined by the amount and type of melanin produced by specialized cells called melanocytes. While the number of melanocytes is relatively consistent across individuals, variations in skin tone arise from the quantity of pigment produced and the specific nature of the pigment granules.12

Melanin exists in two primary forms: dark brown eumelanin and pale red or yellowish phaeomelanin. In individuals with darker skin, there are heavy epidermal deposits of melanin, whereas fair-skinned individuals have significantly less of the pigment in their epidermis.123

When exposed to sunlight, melanocytes increase their activity—rather than their number—to produce more melanin, resulting in a suntan. This process serves a vital protective function, as melanin granules form a shroud over the nucleus of epidermal cells to shield them from damaging ultraviolet radiation. Conditions such as albinism occur when there is a reduced or absent synthesis of this pigment.124