EDIT: Stil not working in Maya, but blender did it with no problems at all. I´d still very much like help, so I can do it in Maya in the future, since I´d rather stick with Maya. (I have no prblem with blender, I am just a Maya lover ;P).
If you like this kind of illustrations, many in D&D were from Larry Elmore. You may remember D&D Handbooks. Basic D&D Handbook Cover Expert D&D Handbook Cover Companion D&D Handbook Cover
You could (in theory) encode all your colours/gradients as a curve or 1dLUT , mark up the various different "material IDs" with vertex color and offset them with parameters on the material instance to index into the curve. bit of a waste of effort though imo :D