You could use the dark gray used on the hair to shade the upper part of the neck that's right up to the chin. Oh well, use your time on your next piece and make something better :D

 Babum, and suddenly the neck is shaded. Although it does emphesize the shading more, which is a good thing, it makes the neck look flat in my opinion...

For 20 minutes, that's pretty good. I can't even get an outline done that fast xD

Anyways, one thing I noticed for the shading is that the entire neck should be shaded because you did a fair amount of shading on the scarf.

 Thanks. I'm actually kinda glad, I'm TERRIBLE with color, but I think I know my monochrome fairly well.

This is a badass avatar. Really like the color usage (...or lack thereof. heh)