Interior color has a much larger effect, particularly on the interior surface temperature and therefore how easily you can get burned on the thighs by leather.
As far as internal ambient temperature, again a darker interior causes the vehicle to heat up faster, but the steady state temperature is higher with a dark interior.
So...as far as internal temp is concerned, only the interior color is significant, and even then the steady state temps aren't significantly different.
(And it doesn't take long on a sunny day for the interior to reach its steady state temp). I'm in Virginia...I'll be going with a lighter interior (probably tan) and the blue, grey, or white.