If you want your swimming pool water warmer is it better to do the interior in a light color or darker color?

Darker. Dark colors absorb heat, light colors reflect. I was told our black bottom pool would keep our pool 10 degrees warmer. Not sure if that is true, as I have no basis for comparison, but that's what I was told by our pool builder and other pool experts. Another perk of a darker colored pool is they hide dirt and debris really well.