a dark bluish purple could also look like an almost maroon colour, again like delta 07. Where else did they have to go? Yellow wouldn't show up against white armor, and Scorch already has yellow mixed in with the black on his armor. Blue could work, but in a dark environment, it might be hard to tell which is blue and which is green. Red is already taken. Green is already gone. A greyish colour would have looked pretty cool.
I still think that you can't get much more mysterious, stealthy or just plain badass, than a nice matte black finish throughout the whole of the armor, and just the faintest glow of red or orange from the visor, just too look a little bit more sinister, especially from the shadows. All you see is a faint red T looking at you before a vibroblade comes between your ribs.
The very existence of flamethrowers proves that some time, somewhere, someone said to themselves, "You know, I want to set those people over there on fire, but I'm just not close enough to get the job done." -George Carlin