Have red indicate the 'background' (You get 255 of them, and it could be used to indicate passability if you don't want to use the alpha channel), blue indicate your 'enemy', green indicate items and then your alpha channel being a binary 'passable/impassable', or if you need a pathfinder, you can treat it as a 'cost' variable. Then, in a single bitmap or PNG you've got everything you need, depending on how you want it.
Ah yes... the alpha channel came to my mind (just in passing... wasn't really sure how it might be used) but I hadn't thought about using the separate RGB values like that.
I suppose that's about the most information you could reasonably put in an image. Shouldn't be overly hard to follow then as long as you keep an organized list of what each colour means.
Thanks for that, I'll have a think about it and see what I want to do. Might keep it simple for now