It's a joke dude. If you look at the costumes worn by Sentai heroes assembled together they look like the colours of the rainbow anyway! ('cept for pink, black, white and clear )

There are probably enough colour variants of Optimus Prime out there to do it too!
e.g.:
red = regular version
white = Ultra Magnus
black = black version (JafCon 9)
blue = Diaclone Powered Convoy
yellow = Shining Magnus