A graph containing a Kempe Chain consisting of alternating blue and red vertices. Kempe Chains like this one were used in attempts at proving the four color theorem.
<a href="//commons.wikimedia.org/wiki/User:IntegralPython" title="User:IntegralPython">IntegralPython</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>