When Light was created, brainy-head chose the colors.
Notes
Prismoo can be leveled up by combining two bahamoots. Prismoo has no further evolution, but has an infinite level up ability.
Trivia
Prismoo and Cowseer are the only ultimate beings that doesn't end with the letter "s" (the others are Cowtlas, Divinitits , Cowroboros and Mooximus).
brainy head is a pun of prism.