well, thats one of the reasons i've posted, its a kicker sx900.4
I was looking at the amp, I'm not sure if the top part is much of a heat sink, I'm sure it does some heat dissipation but I don't know about how much.
Perhaps polecat can chime in ?
OK, I'm not personally intimate with that specific amp, but let's approach this from a common sense perspective and maybe that'll help you with your choice of what to do.
True heat sinks will generally be finned aluminum (or channeled or in some form that increases their surface area). Aluminum is used in place of some other materials because of its easy machining, resistance to damage, and price (although still expensive).
Now, amp makers still want to impress their consumer visually because there's really not all that much to them, so they're given flashy shapes and fancy paint jobs. But, not all of the time the colors you're looking at is paint. It's not even powder-coating (although some is).
Anodizing is an expensive process to getting "color" onto raw aluminum, so amp makers will try to employ that process to as little material as possible while maintaining the "look" they're after, but it's probably one of the best ways to color while not drastically affecting its capability as a heat sink. And, although the non-heat-sink-specific parts of an amp (metal, that is) are not as significant to reducing heat, they are still part of it, and may even transfer from one heat sink to the other via heat conductivity (under the paint/powder-coat).
So, those things being considered, take a look at the factory amp. What part is it that you're wanting to color? On the original surface was it raw aluminum or anodized aluminum? If so, if it were me, I'd strip whatever powder-coating and everything else off of it and leave it raw.
Now, if it was (factory original) painted, powder-coated, plastic, chromed, or anything else that may insulate the surface, my guess is: it's not a significant heat dissipating part of the amp and you're absolutely fine painting it or whatever else you want to do with it.
Does that help a bit?