I really like the shading style itself but something about it seems a little... off.
This is no disrespect to you because your art is really nice, I just can't put my finger on it.
I think it may just be the clashing of styles, at least to me, between how you drew the face compared to how you shaded it. I'd personally have gone for a slightly less cartoony looking style for the facial features (this is just my personal preference) so it could line up better with the shading.
Again I totally need to stress this: this is probably just personal preference and bias rearing it's ugly head.
Since you still have the SAI file available, you could mess around with the face to see if it looks better to you or not.
Colors are nice, but coloring with the airbrush tool doesn't have as much control as say, using the brush tool and then blending out the edges with the watercolor tool. But that's just me. xD I prefer having some defined edges when shading to make things look more.. structured?
Once you finish and add depths of shading and highlights, it would look great. :3
The color is looking every nice so far. I especially like the texture on her hair. The main tip I can think of to anyone working on coloring/shading is to not be afraid to push beyond your comfort zone. Try experimenting with highlights for example: perhaps a soft brush of yellowish tone where the light source would hit her hair.