Science of Skin
Fewer than half dozen genes make a substantial contribution to skin color. These genes are activated in melancyte cells to string together amino acids floating nearby into an enzyme called tyrosinase.
A couple of additional enzymes make or inhibit the making of various pigmentations of melanin, resulting in our skin tone. The action of the enzymes occurs both automatically, under genetic control, and in response to environmental stimuli, such as sunlight and nutrition.