Looking good. :] Here's a little tip though, instead of using pure black to shade, try a different darker colour: blue, purple, etc. Black tends to kill pictures (specially if you're using watercolours, but that's a different story). Same goes for using pure white for lighting. If you use white sparingly, the places that you do use it will really stand out. For example, if you make the "whites" of you character's eyes a shade of blue, brown-grey, or yellow tinged, then throw in a white highlight, they can really pop. Also, try experimenting with different brushes. The airbrush and other solid colour ones can only get you so far.
