I've thought about this too over the years. All the green and yellow and other less-than-desirable colors for the Villains? I believe it was simply because all the good colors were taken by the heroes. Think about it...first you create the hero give them a great color like red or blue. Then when it comes time to develop the villain what are you left with? They cant be the same colors as the hero, and so green it is or maybe next month chartreuse, mustard yellow with green accents. Of course with the Flash being all in Red you could always have a villain dressed in blue, Cpt. Boomerang, Cpt. Cold But after that it seems to gravitate back to green and yellow. It still goes on today, new villain? Lets have his costume green and yellow lol!