If it is plastic, spray paint can be used (as graffiti artists do; apply thinly, possibly multiple times). However, the result will never fully match original dyed-through plastic. If possible, one should try beforehand (maybe there's such a radiator in the basement or elsewhere).
About the material. The sound often reveals a lot. If a magnet sticks, it is metal (the opposite is not a clear indication). If there is a nameplate, one can google or ask the manufacturer (if it still exists). If the cover (?) can simply be removed, one can deduce the material from the weight, then the density.