The sunlight effects the color pigment in your skin. I believe that it is called Melanin. THe longer someone is in natural sunlight, the more their skin produces melanin. This causes people to get darker skin. Melanin is there to help reduce the intake of various harmful sun rays.

Basically, the sunlight is the most natural source of Vitamin D. However, overexposure to it may cause some damage to the skin. Some fair-skinned people have sensitivity to UV rays. And people who have darker skin have higher tolerance to it, or may need to get exposed more so they absorb it better. (Sorry, no racial drama intended.)