What's that cellophane-like stuff that makes lights look rainbowy?

My friend and I once got these little things called 'mood adjusters.' They were little masks and the eyes had some plastic-like, cellophane type stuff in it. When you look at lights through them, everything looks rainbowy. I'm wondering what the cellophane-like stuff is and where you can get it; I'd like to do a craft using it. Much thanks. :)