OK, here's what I was looking for...
Christ, I think what I understand you to be saying is that over time (you tested with 8hrs, EPA says 4 hrs, and it may be as little as 1 hour), the interior temp of a car is going to be identical regardless of the outside color when sitting in direct sun. Makes perfect sense, especially since the car isn't very well insulated to begin with.
I think what Edward is trying to say is that the rate of heating within the car would be higher with darker colors vs a lighter color... so if he is parked in the shade (and the car is cool inside) and then drives out in the direct sun, the interior will get hotter faster if it is dark colored vs a lighter colored car. Eventually though (in 1 hour, 4 hours, whatever), it will still reach the same temp. Likewise, in order to keep the interior cooler, the AC (if used) would need to work harder to counteract the rate of heating in a darker colored car.
One study done by Florida Power & Light on houses indicates that a lighter colored roof helps reduce electric bills... (see
Residential | FPL | Keep the Heat Out and the Savings In) not directly applicable to the discussion at hand, I know... but a similar idea. Obviously not the only thing that'll help... I agree that window tinting and those foil "sun shades" would help a lot as well.