Human
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-coloured skin is two-fold. It radiates less heat, and it absorbs more sunlight. In weaker sunlight a darker body produces less vitamin D than a lighter body. The selection for lighter skin is driven by these two reasons. Therefore, in less sunny areas, natural selection favours lighter skin colour.