Colors of the Chromatics
Summary
At one point in the far past, Tiamat had a red, a black, a white, a blue and a yellow head. And so the first of the chromatic dragons had those colors. One day she and Bahamut had a daughter- a chaotic evil dragon with purple, green, and orange heads. This lead to the creation of purple, green, and orange dragons. Eventually this daughter tried to kill Tiamat, to take her throne. The daughter failed, but did manage to kill her mother's yellow head, who was the most intelligent of the heads and thus in charge. As a part of their fight or out of vengeance afterward, Tiamat swallowed her daughter's green head, incorporating it into her own form. That's why the chromatic dragons include the greens, but no yellows, and why yellow, purple, and orange dragons are rarely if ever seen anymore.