Once a CENT leaves the mint, copper can change to many different colours through environmental exposure, chemical reaction, cleaning etc. It can sometimes be hard to say exactly what causes a colour change.
Agree with all above. Might be fun to hang on to cents which have a wide variety of coloring (toning/staining). I have a Lincoln cent Dansco full of toned cents from 1909 through last year. A number of them may be 'forcibly' toned, but the most interesting examples by year are what I gather.