Is it more efficient to paint a heating radiator black or silver?

The efficiency of a radiator depends less on the type of paint on it as it does the type of metal it is made of. Allow me to use an analpgy of soda. Soda in an aluminum can will cool off in your fridge faster than it would in a glass or plastic bottle. However, aluminum will also be the fastest of the three to warm back up after it is removed from the fridge. This is all relatively basic calorimetery. The the specific heats of 2 metals will tell you which will heat up faster when there is superheated radiator water running through it and which will heat up slower but be emenating the heat into the room for longer. The color paint on the radiator makes a negligible difference.