An apple gone brown on the counter or a penny turned green over years have both gone through the process of oxidation, or exposure to air, which changes their properties — including color.
Oxidation is a word used to describe what happens when metal rusts or starts to disintegrate over time with exposure to air. But the process of oxidation — a word that looks very much like the word oxygen — isn't limited only to metals. Bananas turn brown over time because of the oxidation process. If you see an old car crumbling from all its rust, it's seen its fair share of the oxidation process.