couple studies show that no matter the color, 2 identical cars, parked next to each other will eventually reach the same exact temperature. The only difference is how long it takes to do it.
The studies showed a white painted roof will take 3 hours while a black roof will only take 1.5, for example, to reach 120 degree's inside on a hot day. (
not exact numbers, just an example.)
On a trip where you are in the car for an extended period, (
overlanding), this makes color a moot point.
I think it would still help with the AC on or windows open and air moving through, because you're applying a lot of heat removal that a parked car doesn't have.
On the other hand, insulated roofs like the safari roofs on vintage series land rovers that have a separate skin of aluminium riveted to the roof, with a void between it and the roof itself are VERY effective at lowering the temps by a significant amount.
When I had a maggiolina on my cherokee, on hot 100 degree days, where a normal car may be 100 or more inside without shade, my jeep was literally cool to the touch inside, and air temps were about 10-20 degree's cooler. (
Though 80 degree's isn't COOL per se, we all know that 80 degrees feels like stepping into a icebox on a 100+ degree day)
I duplicated this with my LJ wrangler by putting a solid floor on my roof rack. it completely changed the charactoristic of driving my wrangler, which came from the factory with an uninsulated hard top. On hot days, it was actually refreshing to hop into my jeep, and seldom did I need to turn on my a/c.
just fyi.