Can You save on Cooling Costs with a Lighter Color or White Roof ?

Short answer: Absolutely. Danny Parker, a senior scientist at the Florida Solar Energy Center in Cape Canaveral, conducted a variety of tests on homes in Florida over a period of several years. In the experiments, he noted a reduction in roof temperature of more than 50 degrees. The savings on energy costs varied from 10 percent to 40 percent. And...all this research is from the late 1990's, but more recent studies confirm the findings as well. So much so that the U.S. Secretary of Energy, Steven Chu, asked Americans to replace roofs with white materials or at least paint their roofs white. 

Newer research shows that white coatings on roofs can significantly reduce the surface temperature of the roof. A typical asphalt roof reaches around 80 degrees above ambient temperature, while a white roof only increases by about 12 degrees. The excess heat on the roof seeps into the home, combatting efforts to cool the home. By keeping the roof temperature down, you save quite a bit on your cooling costs. You can save between 15 and 20 percent on your bill.

See this article for the details of the 1990's experiments. It includes full details of each house, graphs of temperature readings, and estimates of the energy savings.