Copper stains can be taken off with anything that's acidic. Straight acids are the best for removing these stains; however, household chemicals are also sold for this purpose.
Copper can get blue or green stains, due to oxidation. Moisture, temperature and the level of pollution all cause copper to oxidize. The stains are only on the surface and don't cause the material to become weak. It becomes harder to remove the stains when they're present for a large amount of time, because they settle in the material. One should avoid mixing chemicals that contain ammonia with bleach, because this combination results in a chemical reaction.