When Keith Rule and a team of interns walked onto the roof of the main building of the Princeton Plasma Physics Laboratory one sweltering day last summer, they could feel in the soles of their feet the temperature difference between a black section of the roof and a white section.

The white roof was tolerable but the black roof was too hot to stand on. “You could feel it coming through your shoes,” recalls Rule, a senior project engineer at the lab, who is collaborating with Elie Bou-Zeid, a professor of civil and environmental engineering at Princeton University to test the effects of the roof colors on energy use.