The increase in pigmentation or coloring in the skin is a combination of the Ultra Violet (UV) rays that come from the sunlight and the amount of melanin within our skin. The sunlight will hit the cells of our skin. Immediately after that, when it hits our skin, it causes for our melanin (or the amount of coloring already within our skins) to increase its production. Each individual reaction and sensitivity to UV/UVA rays from the son will eventually determine to what extent the pigmentation of the skin will darken. Some people become really dark while others just turn out reddish instead of dark. Eventually everything will depend on your individual response to the rays. Do not forget also that excessive exposure to UVA rays is extremely dangerous and may cause that same pigmentation process to turn against itself and produce cancerous cells.