rationalskepticism.org

There is some statistical evidence that those with darker eye colors are more likely to suffer from SAD (seasonal affective disorder) than those with lighter eyes. Also, there is some evidence that the genes responsible for eye color are linked in some way to those controlling skin color, but the short answer is no one really knows, yet, what selection pressures were active. I'd imagine others here have more information on it.

http://www.wonderquest.com/eye-color-evolution.htm