It was widely reported back in the day that "Grumpy" Jenkins painted his race engines white to contain heat that would then be converted to mechanical energy. I am of the paint is a better emitter, but think the question of how much insulation paint adds, is a valid one, and the comment on black lacquer is on target.

An example that comes to mind is the enclosed glass bulb with a two side free spinning black sided/white sided flag. When lit by a light/heat source, the black side flag spins away from the energy source, indicating I believe it heats more, and gives up its heat faster, not sure if this is a valid example of anything, but hey this is moparts.