I just painted a huge heatsink a few days ago. I used one coat of grey Rust-Oleum "automobile primer" and then one generous coat of silver Rust-Oleum "hammered" finish. It turned out better than I expected.
This amp is being changed from class AB to class D so I'm not too concerned about any dissipation loss.
If it is flat black and you need to use paint, try the bumper coat black. Stuff works really well and you can vary the texture and shine some. I have done some end plates and bottom covers that looked great. I sanded with 600 before paint. Should not have to use primer with it if careful, I'd recommend you go thin as possible with any kind of paint and strip it bare as possible if a heatsink.
Yeah...outside like say for example, one of the MRP Alpine amps, a MRP-M450 or MRP-M650...The outside is black with the silver strips on it...What if you took the silver strips off and painted the black a different color....Wondering if that would make a difference....
The whole thing is the sink, but IMO I think it would not matter that much on one of those if it was installed with good air flow. If the aluminum is still bare and you make the paint on the rest a little thicker I doubt it would change it that much. It would depend on how it is used/installed much more I would think. Also class D amps tend to make less heat than AB amps to start with. The M450 is pretty small though, it may get warmer not sure.