You need white paint that works for infrared as well as visible light. But along those lines, instead of mounting solar panels, just mount something cheap to shade your roof.
And if someone figures out such a magical paint it's going to be an great business idea/overnight success story because you could put it on cars and reduce the afternoon-oven characteristics.
The body of a car is pretty insulating. The windows are the problem when it comes to heat.
And make sure it's properly insulated to keep the heat that is not reflected outside