I would submit an un-painted engine would dissipate the most heat as it's one less layer of insulation on it. However, it'd corrode, especially if aluminum. Color doesn't really matter when it comes to heat, it's infrared radiation, not visible radiaton you need to worry about. Besides that, most engine heat is conducted out through coolant, not radiated from the block. Most electrical heatsinks that have to do a really good job aren't painted at all, the paint layer would insulate against heat being transferred from the component to the heatsink and again insulate it from radiating out of the heatsink. The only time they'd be painted is when made larger than needed so any loss in efficiency would be more than made up for by corrosion resistance if needed. Look at any of the higher level passive cooling solutions for computer cpu's and video cards, almost all are unpainted. The only thing on them is thermally conductive paste to fill in any gaps on the surface of the IC and heatsink (any air gaps will insulate) and nothing on the exposed areas.
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