I did a quick paintover! I upped the saturation a little bit to make those colors pop! After that it was a matter of looking at the values and where the light is coming. Then, with a multiply layer, I just went over the whole piece and shading, always thinking about the light source. Where does it hit? where doesn't it hit? Not shying away from going darker with it, to really show where that form turns away from the light. I warped it around a bit aligning the eyes and making them rounder and extruding that secondary arm to add even more to the silhouette. Lastly I added "life" to the skin by brushing in some red in on that skin on a soft light layer, mostly in that area where light and shadow meet! Hope that helps! Great work keep going!