The artist says about this:
Clarke is yellow because she represents the sun, stars, hope, and optimism. Lexa is red because she represents blood, war, and passion. The blue on Clarke represents the sky because she is leader of the sky people. The green on Lexa represents the ground because she is commander of the tree/ground people. The blue is radiating from Clarke's heart because she makes decisions with her heart. The green is radiating from Lexa's head because she makes decisions with her head.