you can fake that for the time being by duplicating your mesh, using a push modifier (if you're a max user)to push the duplicate out a little, then invert the normals of the pushed mesh, and give that mesh a black material.
I feel like your handpainting technique is moving the right direction - your biggest issue right now is the lack of uniformity in the texel tensity. Push the colors, push the shapes, push the accents and edges. exaggerate!
my advice would be 1 - smooth, push, move, smooth push, move, smooth... push... move... 2 - bring the mesh to max/maya and move those verts of the base mesh, fix in zbrush after importing it.
I have seen in some tutorials that it's not good to manually move the vertices of a cage, so all i do is duplicate the triangulated lowpoly and adjust a push modifier until the cage completely covers the highpoly, but there are always some parts or details of the highpoly that are further from the lowpoly so in those parts…