Why are people in cold climates darker?

i was always told in school that evaluation explains why people are different colors in different areas... ok so supposedly people closer to the equator are darker because it is hotter and when people moved away from the equator they got lighter because it was colder.. but the only issue i find with this is Eskimos are darker then europeans and they have lived on ice and snow way longer than people in europe... europe is no where near as cold as where the eskimos live... why didnt they get lighter when they started to live in snow with hardly any sun what is the deal here.... any body know the answer???