Bookbinding Colors

Hi Guys, I want to bind the Empyrean books. I thought about just using the Dragon Colors for the Covers (i thought there where 5) but there 6. Dragontalons also 6. Quadrants? Four.

But there will be five books... Can somebody give me a hint for a 5 Colors i can use for this series that also have a meaning? I couldnt really think of anything....