I think theres a plug in for PS that does the red/green job for you.

I had a look into it, very briefly, as it was one of the "killer tips" in, well the maya Killer Tips book! But never got round to giving it a go.

If you want i'll scan it in and mail it to you, though it wont be unitle the new year as the books a t work, but its there if you want it (and/or you've not got the book, though its a good buy, it gives 1/2 paragraph short, tips for setting up, modeling, lighting, rendering and scripting)