My wife and I are seriously thinking about getting the roof of our house painted white (for energy efficiency and environmental reasons). And today the New York Times has an article on the “White Roof Movement.”
Money quote:
Studies show that white roofs reduce air-conditioning costs by 20 percent or more in hot, sunny weather. Lower energy consumption also means fewer of the carbon dioxide emissions that contribute to global warming.
Prometheus Unbound 
Everybody is aware with the fact that darker materials absorb more heat from sun than white/light colors, but an interesting data I want to share that black surface in the sun can be 40°C (70°F) hotter than the reflective white surface. This phenomenon occurs in the case of roofs also and heated roof then transfer their heat to surrounded air and contribute to heat island effect, while reflective metallic/ ceramic roof can reflect 65-75 %solar light. According to California energy commissioner “White roofs can cut a building’s energy use by 20% and save consumers money,” and “The potential energy savings in the U.S. is in excess of $1 billion annually.”
http://buildinggreenlife.wordpress.com/2009/08/17/magic-of-white-roofs/