Hi Gray! Very good painting, and an interesting idea to use photoshop over greyscale for coloring. Never thought of it before. I also don't think your wizard looks legless. Before I read your comment, I thought he was kneeling down on his knees
I don't know of a way to do what you want here... unless you want to use a pen-shading type of style (similar to what is often seen in watercolor paintings) and then use oils to paint in a layer beneath that. A comic-book style of shading could be done this way too. Probably not quite what you're looking for, just throwing ideas out there for greyscale shading and then adding color
"If a man does not keep pace with his companions, perhaps it is because he hears a different drummer. Let him step to the music which he hears, however measured or far away."
-- Henry David Thoreau