system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is there more diversity in hair/eye colour of caucasions compared to darker coloured people?

Why is it so uncommon for darker coloured people to have anything but brown hair/eyes and what varies when it comes to caucasion people?
gpt
ELI5: People need different traits for different climates. The genetic mutations that made it possible for people to have lighter skin in colder areas with little light, made it more likely that genetic mutations would occur in other coloring (such as hair and eye color). It was never beneficial for people in warmer/bright climates to have lighter skin, so they do not have the genetic mutations that make it possible for it to occur. 

In cases where a blond person has children with a brunette, the genes for darker coloring are dominate so the brown eyed/brown hair parent has a better chance of passing their coloring to their child. Since people with darker skin usually don't have those light coloring mutated genes hanging around their gene pool, it makes it even more likely that light coloring won't occur.