Ironically, the color of an actual bike shed matters. If you paint it dark, it will be hotter inside when the sun shines on it compared to it being light colored. On the other hand, bright colors that stand out against the surroundings attract unnecessary attention to the existence of the bike shed. Ideally we want it to be inconspicuous and to convey the idea that it contains nothing of value.

Obviously, the correct coating for a bike shed to meet these requirements is in fact a cladding: of externally facing mirror panels. Except that if they are ordinary glass mirrors, there is the obvious vandalism that may be inflicted when some rascals do actually bespy the nigh invisible structure.