Regarding paint color, i did a project for an engineering class last year, designing a heat sink. Flat black almost always has the highest emissivity, so the thinnest coating of the flattest black is going to be the best for emissivity, and thus radiant cooling. I did find out one odd thing though: rusted cast iron has damn near the best emissivity that i could find. Kinda annoying when my pa50's cylinder is rusted to all hell, and looks horrible, but works perfectly!