Hi Willtre, I am far from an expert in painting, but I think that one of the best ways to get a shadow is to use complimentary color. Not that you would paint the shadow side of an apple green, but you should mix green into the red. It may be tough in ArtRage to do that (or not?) but how about glazing over the color on a seperate layer. I think that red over green would appear "warmer" than green over red. Shadows are tricky in that they will have reflected light in the from whatever they are sitting on or near. they are also darker clost the object and lighter as they move away, and the edges of the shadow blur more the farther away they get.
I think that the best advice is EXPERIMENT!
Give a man a fish and you feed him for a day - Teach a man to fish and he will sit in a boat and drink beer all day! Joe V.