Why are most radiators painted white?
Purely because it matches better with white walls. There won’t be difference from a white radiator as a black radiator – it is just an outer layer of paint. The myth comes from black body radiation. Black bodies absorb radiation better than white bodies, however they also emit radiation better than white bodies. So if the rule of black body radiation did apply to radiators, a black radiator might infact emit more heat than a white radiator as it will absorb the heat from the hot water better and emit it to the surrounding cooler air better. However radiators work moslty by conduction and convection and not radiation so this would not happen even if the radiators were true white or black bodies.
