This is my first time attempting a light scheme this complex and I'm pleased with the result so far. Feedback welcome, I'd like to fix anything early if some kind of mistake slipped in!
Cast shadows were made using a "classic" shortcut - you render first, assuming light hits the entire face and considering only AO (which is a bit non-intuitive, but when you paint a portrait that's lit in diffuse light, you essentially get that). Then you duplicate that layer, darken it and shift its hue, and use a mask to apply it only where you want it to cast, in this case the face on the shape, under the nose, neck, and under every floaty bit. Then I merged the shapes and kept at it with overpainting, notably adding a lot of sub-surface scattering. Hope you like it so far, but feel free to leave feedback!!