Not their function, but what ARE they? What are they made of? Is it really gel? More importantly, can i make one out of a piece of translucent colored plastic sheet?Or do the real gels have some unique properties that a plastic sheet doesn't have? Normally i Google things like this (read - easily DIYable at first glance), but all i can find is where to buy them and how to get samples, which are not available where i live. Thanks.