This is only partially correct. Blaylock is lumping emissivity and reflectivity together, which is a problem since they are opposites of one another. Yes, efficient radiative cooling requires a high emissivity, but that means it must necessarily be a great absorber as well - and likewise a poor reflector. In the case of visible light, the best emitters are black. However, since the radiation from a spacecraft will be IR (you've got other problems if your spacecraft is hot enough to emit visible light), the heat sink could be painted pretty much any visible color - only its IR color would matter. That said, silver is pretty much the worst choice you could make, since silver objects have pitiful emissivity (in both visible and IR), due to being great reflectors. Dull gray would be okay, and would save on the weight of paint.