The external color does not significantly affect how much the inside of a car heats up in the sun, says Sanford Klein, director of the UW-Madison Solar Energy Laboratory and professor of mechanical engineering.
Cars warm up in the sun due to the greenhouse effect: Sunlight passing through the windows into the car is mostly absorbed by interior surfaces, then radiated back to the air as heat. That heat does not pass back through the glass, which is an effective insulator for radiation, and the inside temperature can rise above the outside temperature.
“The visible radiation that we see is transparent to the glass but thermal radiation is not and gets trapped by the glass,” Klein says. “As a consequence, the inside of the car will warm because radiation is coming in but not much is going back out.”
The interior color may make a small difference in internal temperature, he says, because darker internal surfaces will absorb slightly more solar energy than light ones do.
Tinted glass will have an even more pronounced effect, he says, by reducing how much solar radiation enters the vehicle in the first place.