You are not really serious are you? Paint will just holds heat in, on the shroud probably won't make much of a difference either way but on the radiator, it would just make it worse.
They call it a radiator because it RADIATES heat. There is a big difference in the amount of radiation based on color and temperature difference. Paint conducts heat and then radiates it from its surface depending on its color. If air is hotter than the surface, a black color then absorbs the heat while a white color reflects more heat.