distinguished from each other by coloring
... that six or more keys on a keyring can be distinguished from each other by coloring the keys using only two colors, but rings of three, four, or five keys require more colors?
... that six or more keys on a keyring can be <b id="mwBDA"><a href="./Distinguishing_coloring" id="mwBDE" rel="mw:WikiLink" title="Distinguishing coloring">distinguished from each other by coloring</a></b> the keys using only two colors, but rings of three, four, or five keys require more colors?