Does the paint colour matter on a heat sink?
Do not paint heat sinks. The layer of paint will act as an insulator between the metal and the air, reducing its ability to dissipate heat.

Anodizing a heat sink is less of an issue. The anodization layer is much thinner than paint (a few micrometers in aluminum, for instance), so it presents a much lower thermal resistance than paint would.

The color of the paint will not have a significant effect on cooling unless different colors of paint are thinner or thicker than others, or if the heat sink is exposed to direct sunlight.