Amanita muscaria - heck of a good-looking mushroom, isn't it? Nice bright red? In fact, as I was hunkered down over my camera and tripod trying to get a decent shot, a nice woman happened by and was amazed that there were such things as red mushrooms. O.K., now that you mention it, a mushroom with colour is sort of an unusual notion. In plants, the colour is derived from organelles known as plastids. Generally, these plastids contain pigments that are involved in photosynthesis. Fungi don't photosynthesise, so what are the pigments for? Well, I suppose one good reason could be the fact that mushrooms are how some fungi asexually reproduce themselves. It would probably be a reasonable idea, in these cases, to discourage any animals that might think, "Hmmm, looks tasty to me", since consumption would kind of put an end to the whole "reproduction" program. What better way to say, "Hold it right there, pal. Eat this and you're gonna die." than to indicate this with a bright, consistent colour? Not that red is the universal "Danger!" colour. Any colour would have done just as well (provided the most likely predators could see it), just so long as it was consistent.