Any conventional paint you put on them will considerably reduce their cooling performance. The "paint" on most RAM sinks is anodization and if you want to change the color, you have to strip the existing coat and re-anodize with different chemicals.
Unless you happen to have a high-voltage, high-current power supply, the various acids for stripping and cleaning, the salts and other chemicals for anodizing, and other related supplies, your best option would be to bring the heatsinks to an anodizing shop to have them re-done.