Unpainted should shed the most heat due to one less layer insulating the engine. The reason you don't see shiny heatsinks is a surface area issue, not one of color. A shiny surface has a lot less surface area than a rough surface, so it willl not work anywhere near as well as a heatsink. Most passive (read: no fan or anything blowing on it) heatsinks used in computers on the parts that put out the most heat (cpu and gpu) are unpainted. The less important ones may be annodized but are not always black, though that's probably the cheapest color.
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