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There are two large factors that will influence the external temperature of the vehicle for a given surface coating, reflectivity and emissivity. In a nutshell, reflectivity is how much of the energy striking the surface is reflected back versus how much is absorbed. Emissivity has to do with the amount of energy that is re-radiated back off of the surface as it heats up. (Yes I understand this is a very simplified explanation and that is is actually more complicated.)
Even for darker paint colors, the overall reflectivity may be different due to reflectivity coefficients in non-visible spectrums. (E.g. cool roof shingles that aren't white.) The other complicating factor is that even if a material has high reflectivity if it has low emissivity it may reach a higher temperature because it can not shed heat as effectively. That is why some specialty coatings or polished metals are actually worse than a typical painted surface.
The other factor to consider is how much the temperature of the outside surface of a car impacts the temperature of the cabin. This is where insulation and/or ventilation comes into play.
This hits home with me, as my new (1999) 4Runner is black, which means I am at a reflectivity handicap. Don't really plan on painting this ones roof white. I have some peel and stick foam, which I plan to apply to the interior of the roof surface to slow the thermal transfer of heat into the cabin. I also plan to put thinsulate in between the headliner and the roof for the same purpose.
Being able to park the vehicle inside or under cover makes a huge difference obviously. A solar powered fan would be ideal for ventilation, but cracking the windows (assuming low chance of rain) can also be effective. Simple things like opening all the doors for a minute or two prior to getting in the vehicle can also make a huge difference in comfort and how quickly the AC can catch up.
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