The paint color does not really matter. If you have 2 heat sinks one is polished and one is painted or anodized. The painted/anodized one will dissipate the heat faster/better than the polished one. Lightly painted not gooped or slopped on. What the paint/anodizing does is raise the emissivity of the heat sink(head) and thus is dissipates heat better.
Anodizing is better than painting.
Thermally speaking,
Chuck