As it turns out, skin color is almost completely a function of how much UV sunlight an ethnic group receives in the fall. More sunlight means darker skin. (Check out this paper by anthropologist George Chaplin for more details.)
Which is why a lot of the traditional talk about race and skin color is just plain dumb. Human beings are all one species. Move your family to a sunnier spot, and your skin will change color after a few dozen generations, too. In other words, skin color actually is just skin deep.
...and here's what the global mean temperature looks like, for comparison: