[Christ]

NachtRitter - My primary concern in saying that the paint really doesn't matter is that the glass transmits more heat energy into the car than the metal ever would. Glass transmits light into the cabin of the vehicle, and since there is a large surface area of clear glass to act as an intensifier for the light, the vehicle's interior surface will heat much faster as a result.

It's the interior surfaces absorbing the heat from light energy, not heat radiance from metal panels.

If the color of your car makes any difference at all, it would be an extremely negligible difference, at best. Possibly a few degrees over a 2 or 3 hour period, hardly something worth painting your car over.

Think about how efficient heat transfer can be:

Glass transmits light, into the surface area of the vehicle's interior,
The surface of the interior upholstery absorbs the light energy as heat.

Light hits metal painted panels, which absorb that energy as heat.
While the temperature below those panels is different from the temperature of the panel, heat radiates slowly into the vehicle.
As the temperature increases, the differential decreases, and heat transfer slows. Eventually, the interior temperature is equal to the temp of the metal surfaces, and the surfaces begin dissipating heat into the open air.

Obviously, black absorbs more heat than white, but I don't think the light absorption occurs any faster. (I'm not sure, correct me if I'm wrong, and show a source.)

In any event, the windows are the primary source of heat being absorbed into the car, indirectly via the larger surface area of the upholstery inside the car.

That was the whole argument in a nutshell, by the way.