You have a pretty distinct style! I think the composition is adorable ^^
Uhmm I'm not an expert, so please don't take everything I say seriously, especially if you don't think it seems right. I'll do my best to critique. I do not mean to offend you in ANY way and I hope I'm not too nitpicky.
Please bear with me >_<;

Her head seems a bit too big? (I hope that didn't sound too rude ><;; ) Also, her head seems a little flat, and it's NOT because it's not shaded. When you draw heads, try to imagine a skull, and plug in the eyes, sculpt in the nose, and mouth like that. Uhmm, I know I'm not that good at it either, and I really suck at explaining. Try looking in magazines and copying some faces, and maybe that'll help.
I like how her hair follows the laws of gravity, no problems there XDD
Her neck to shoulders need a bit more structure to them. I know my version isn't perfect, but please bare with me >__<;

See when the neck connects to the shoulders? There's a slight bit of slope, it's not a full 90 degree angle, and she needs a little bit of collarbone.
Ugh, I got really lazy with the costume, I'm sorry >___>
I can't help you with coloring, but I can suggest that you can use darker colors for your shadows, and might brighter colors for your highlights. Don't be afraid to have super dark shadows, a really big problem when i first started coloring was that I didn't make shadows dark enough, and the entire thing looked like a big blurry mess x___X
Other than that, you have a pretty good idea of where shadows can go, but there are still some spots that need more shadow ^^; I really like how you shaded her hair as well
biggrin Keep on practicing!
Erk, I really hope I didn't sound rude, I really do not intend to sound rude.
I hope I helped a little.