Skin Color

A person's skin color is determined by the amount of melanin in the skin, which is the skin's pigment. The greater the amount of melanin, the darker a person's skin will be, ranging from nearly pure white to almost black. Some people, known as albinos do not have any melanin in their skin, causing it to be actually white, with only a pinkish tint due to the blood in the skin.
Melanin is present in human skin because it helps protect against an excess of harmful UV rays. As a person's skin is exposed to the sun, more melanin is produced, causing a darker overall pigment, and therefore the appearance of a "tan". If a person is wearing clothing such as a t-shirt, the melanin will not produce more in that area since it is already being protected from the UV rays by an external source, thereby resulting in what we commonly know as a "farmer's tan".
Variations in skin color date back to early evolution times, when humans hadn't yet evolved from chimpanzees in Africa. When the standard light-skinned dark-haired chimps started to evolve and lose their hair, over time and evolution their skin became darker to help protect against the very UV intense region of Africa. However when humans eventually began to migrate further north where the sun exposure was less intense their skin evolved back to being more pale, which would allow them to absorb the needed amounts of vitamin D. This biological and evolutionary process is what skin color is grounded upon.



sources:
http://en.wikipedia.org/wiki/Human_skin_color