Even dark skinned people can get a sunburn, making their skin even darker. Take away the sun and their skin color returns to the original shade. To understand how this can happen, let's take a very basic course in genetics. Genes in the DNA carry the instructions on how to make each and every feature that makes your body. Melanin is the stuff that also gives your hair and eye color.
Genes come in matching pairs.
If you get B from your father, and b from your mother, you will have brown eyes. If your father has B and your mother also has B , you will also have brown eyes. If you get b from your father and b from your mother, there are no instructions for melanin, so you will have blue or non-brown eyes. Now to get a little more complicated. Skin color is determined by at least two sets of genes.