The best colour for a visible “radiator” is whatever fits in best with your decor – since its reflectivity in the visible spectrum has no effect on its utility as a heating (or cooling) device.
Actually, most home heating (or car cooling) “radiators” transfer most of their heat by conduction (to the air) and convection, but even the efficiency of direct thermal radiation depends only on the “blackness” in the infrared part of the spectrum – which can be high even when the radiator is perfectly white in the visible part.
Confusingly, home heaters that rely primarily on radiation are generally not called “radiators” but “radiant heaters” and they typically consist of a heating element and a shiny reflector (which is definitely NOT black!). The heating element (which actually does radiate) is typically some kind of metal coil on which any paint would quickly burn off but whose efficiency may be somewhat enhanced by the fact that it usually has a dark oxidized surface layer (which probably reduces its reflectivity and so enhances its emitivity in the IR as well as the visible part of the spectrum).
Source: (1002) Alan Cooper’s answer to Why is black the best colour for a radiator? – Quora