I like her colour scheme, a lot! It's soft and dreamy, kinda like her expression!
Try to use more vibrant colours when you shade. Like for her outfit and skin, don't just use a darker version of the same colour. Try using a purplish red for the pinker parts of the outfit and a very light pink to shade the white parts. For the skin, try using orange or maybe even a pink to shade.
Also when you shade don't be afraid to use hard shadows. Hard shadows occur when you have something directly cut off from light. So a plane change or if you have an object in from of another object. Use hard shadows when shading arms and legs coming from sleeves and skirts, under hair, under necks and noses, and under sharp folds in clothes.
Shading hair is often the toughest part to shade. I'm not to great at it myself, but there are plenty of tutorials here on dA that can help you shade hair.
While you're working out your colors, I'd suggest adding more contrast and hard edges. Currently it's very soft and "pillowy", which is good for certain things but most objects/scenes have both soft and hard shading. This is an intimidating process for a lot of people, myself included, but it adds a lot more depth and texture to a piece. You have a few hard shadows (her hair, on her collar, etc) but they're very close in color to everything else. Try pushing those contrasts a little more and I think you'll find it adds a lot - eg. hard highlights in her hair, a hard shadow below her arm, more shadows on the hair behind her neck.
I know you can do it because what you did with the undershirt coming out of her collar is really successful! I'd love to see you take that further. It has both texture, some color contrast, and definite shadows so it really "pops" compared to the rest of the piece. Keep going!