Wow! You got some strong lineart going! Your colors are very muddy though. I think it's happening because you're using a darker value of the same color as your base to shade. This is especially bad for skin, as it makes the skin seem lifeless and plastic.
Shadows aren't just black, they have color. It is usually the complimentary color to the base color. Another thing to keep in mind is that shadows are usually more saturated than light.
http://www.itchstudios.com/psg/art_tut.htm <------- I'd recommend reading this.
Hopefully this helps and isn't too confusing!