What causes hair to change color in the sun?

I assume you are referring to permanent changes, rather than just the fact that it might look different. The latter is easy; it's because the sun has a different spectrum from artificial lighting. The permanent changes are because of sun-bleaching. A lot of light will be absorbed more by dark molecules than light ones. The more light, the hotter it gets. The hotter it gets, the more it wiggles around. The more it wiggles around, the more likely it is to break apart into different molecules. Some might be lighter. For the darker ones, just repeat the cycle.