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, and roofing materials have advanced quite a bit since then. See this article for the details of the experiments. It's somewhat dated, but includes full details of each house, graphs of temperature readings, and estimates of the energy savings.