In the few times I have judged stunt, I have never really noticed the paint schemes. When I flew R/C, especially gliders, the bottoms were covered with Platinum Monokote. It is kind of a dark color that showed up real well at altitude. Also it didn't disappear like some colors when you get high up.
But, as someone has already stated, the bottom is usually not as fancy as the top because the top is the part of the plane seen most of the time. DOC Holliday