You can't anymore. Its been removed because you can camouflage yourself a little bit in very dark levels. Just as green is greatly camouflaged in green levels, dark_red in places with lots of mud, and gold in sandy levels, and of course grey in castle levels, etc.
I really don't understand why you put in a differen't color function if there's no variety in it. Lets see, red&dark-red, green&none, gold&yellow, blue&darkblue, and pink&beige, and not to forget silver&white&grey. Don't you think all these colors just look too much like each other? You can say beige, black, pink, and purple are the only real colors without a clone. Where is the rainbow color? And what about the aqua blue, lime, and brown colors?