In natural convection a black or dark colored heatsink will perform 3% to 8% better than an aluminum heatsink in its natural silverish color. This is due to dark colors radiating heat more efficiently.
In forced air applications, surface color does not increase a heatsink's performance due to the increase in convection. A color would be for cosmetic purposes only.
Basic Heatsink Temperature Calculation
If you have a 10°C/W heatsink and you dissipate 5W into it, its temperature will be 50°C + ambient temperature.