What would reflect more of the sun's heat: a polished stainless steel surface or one painted white? Why?
If the stainless steel is the more reflective why is the overall color darker, i.e. grey instead of white?
Also, on a sunny day, which do you expect would be hotter to the touch: stainless steel or the same painted white?
The mirror is an efficient reflector. White absorbs all but re-emits some, less efficint as a reflector.