Heat sink color
If your heat sink will work in the air flow of a fan, the contribution of radiation will be extremely small, compared to the effect of conduction. So, it's best to leave the aluminum bare, as any layer of black paint, with its lower thermal conductivity, will hamper the conduction of heat from aluminum to air more than it may help by increasing radiation.
On the other hand, if you bolt your transistors to the back panel of a box, by all means paint that panel flat black! A flat panel dissipates more heat by radiation than by conduction, and here a flat black surface helps a lot! But it helps only if it looks at other objects that are dark, and cooler than the panel, or if it looks at free space. If you place such a black heat sink in the sun, it will absorb heat rather than radiating it, and get very hot! Likewise, placing a black heat sink inside a shiny aluminum box is useless, because its radiated heat will reflect back onto itself. For that reason, paint the inside of aluminum boxes flat black too, so that the electronic parts inside the box can cool themselves by radiation into the aluminum box!
Do you want another table? Well... here is one about the radiation constant of different materials. This is expressed in (10-8)W/(m2 K2) , at 20°C.
Perfect black body: 5.67
Matted steel: 5.4
Matted zinc: 5.3 (that's why zinc roofs get so hot in the sun!)
Oxidized copper: 3.6
Polished copper: 0.28
Matted aluminum: 0.4 (that's why aluminum roofs are much fresher in summer than zinc ones!)
Polished aluminum: 0.23
Polished silver: 0.17
