The six colors are also mentioned in other places in the game. The elemental order in which you fight the first version of Time Devourer (I think it's actually the Dragon God. Good job, Square translators!) is exactly the same which you need to power up the Chrono Cross: Yellow, Red, Green, Blue, Black, White, CC.

But yeah, I think Chrono Cross is a very rushed game especially on Disc 2.