Human skin colors vary greatly. They can be a very pale pink all the way to dark brown. There is a reason why people in tropical areas have dark skins. The dark pigment (melanin) in the skin protects them against ultraviolet rays in sunlight. The damage caused by UV rays can and does cause skin cancer in some people. Therefore, in more sunny areas, natural selection favors darker skin color. Sun tanning has nothing to do with this issue, because it is just a temporary process which is not inherited. In colder climates the advantage of light-colored skin is that it radiates less heat. Therefore, in less sunny areas, natural selection favors lighter skin color.