Clearly 7, because 7 is the most powerful magical number. That's literally the level of Newton's justification, who invented the concept of a 7-colored rainbow.

While we're here, one of my favorite unsong quotes, at roughly the same level of woo:

    > Then there was light.
    > 
    > Beautiful, multicolored light, ten colors, the seven colors of the earthly rainbow and the three extra colors you only get in Heaven. Ten colors corresponding to the ten sephirot and the ten fingers and the Ten Commandments and the ten digits of the number system and the ten pip cards of the Tarot and all the other tens in all the correspondences of the world.