Copper bathroom sinks can be supplied in a variety of color finishing. They can by dark, light patina, green oxidized, custom made or polished. All of those copper bathroom sinks colors are protected with clear varnish. It prevents further patina darkening. Some bathroom sinks are made of natural copper and those are subject to natural discoloration. In case of those sinks you can keep it dark what is taking some time to occur or clean it occasionally to bring copper original bright color.