I think he was going up against a villain who attacked with colors. He had to switch to a different color costume every night because the lasers the villain was using didn't work on a suit of the same color. eventually Bats got the better of him with the rainbow suit
actually he uses it in The Brave and The Bold againts some rainbow creature wich can fire color lasers but wich have no effect on things of the same color as its laser so Batman came with this "clever" idea to make this suit
XD I've read the comments explaining the rainbow one, but I honestly thought you put it in there so he could go to gay pride parades! The looks on Dick's and Jason's faces make it seem plausible enough.