Yeah, sometime or another, turn off your light, leave the room for about 10 or so minutes, then turn the light back on and look at your little RBP's, I think they'll look quite different color-wise.
But yea, I would't really even bother feeding them shrimp. But if you do, then definitely take off the shell. I mean I wouldn't even feed shrimp shell to my wolffish, so I definitely wouldn't feed it to a piranha.
Just my opinion though I guess.
Good luck with them piranhas buddy, and why don't you post some pics up??!?!