Yeah the green is the closed cell foam which is much denser than the standard white open cell foam. The closed cell being denser makes it much easier to carve more detail into the foam itself, plus when you hit closed cell foam with a heat gun (to give it texture) it looks nice, where as white open cell foam will just shrivel up and melt too quickly. Usually most the closed cell foam that is safe to use is either green or purple so those are the two that are used the most for backgrounds. You just want to stay away from any foam that might have any additives in them, or the ones that have a reflective material on the back of course. Some of the white foam such as used for packaging electronics can be used with good results as well as that is usually a denser foam.
