It could simply be that black paint is cheaper… But I believe that the external coils on the back of refrigerator are painted black for the following reason: There is a rule in nature that says “a color that is better at absorbing heat is also better at radiating heat.” 

The rate at which a body radiates (or absorbs) thermal radiation depends upon the nature of the surface as well. Objects that are good emitters are also good absorbers (Kirchhoff’s radiation law). A blackened surface is an excellent emitter as well as an excellent absorber. If the same surface is silvered, it becomes a poor emitter and a poor absorber.

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