White, as compared to other colors, will reflect heat but not as much as many think. Many white surfaces are not "glossy" and reflective so heat is still absorbed. It's generally considered that a white surface with an "80 gloss" (reflects 80% of the light that hits the surface) is best for heat reduction.
If one really wants to reduce heat on the roof one could spread non-synthetic fabric tarp on the roof and have a sprinkler/misting system that keeps it moist. The evaporation will reduce the temp greatly.
Here's how some keep the roof of their vehicles cool in hot climates:
