And white cars can be upgraded with the creamy metalic whites and light yellows of today. As a conservationist, I like the creamy light green of recent Jags.

Having owned both silver and black cars in Texas, I definitely noticed a major difference. Berkeley Lab’s has published a study confirming your suspicion scientifically, and suggests that exterior color can change MPG by up to 2% due to air conditioning loads (i suspect far more than that in very hot climates and far lower up north).

From the article in autobloggreen:
“Researchers in Berkeley Lab’s Environmental Energy Technologies Division have published a new study that suggests cooler-colored cars could be up to 2 percent more fuel efficient than darker-colored ones. Their research shows that cooler colors like white and silver can reflect as much as 60 percent of the sun’s rays, while darker colors like black only reflect around 5 percent. This means that after sitting in the sun for an hour, a silver car’s roof is a full 45°F cooler than a black car’s roof, which, according to the researchers, would equate to a 9 to 11°F difference in cabin air temperature.”