Looks to me like you could improve the order in which you do things.
I can tell just by looking at your picture that you did the outline first, then filled it with the base color. Then you put down the darkest shadow first, then the lighter shadows, and finally added the highlights last, thus having boxed yourself in a small area ( with the highlights that is ).
There is a much better way to do this. It's as follows:
0. Just choose 3-4 colors to start off with. Darkest / lighter / even lighter / lightest. You can make the in-betweens later.
1. Get your basic shape down, the simpler the better. You can use outlines or blobs of color. Blobs of color are easier for pixeling in my experience. Use a darker shade for this.
2. Take your basic shape and add some definition so you can make sure you got the look you want. You can do this with a darker color if you want.
3. Start adding highlights with the lightest color for even more definition.
4. Now tone down the highlights with the second lightest color, clean up the image.
5. NOW you can add things like the strings and outlines if you want. If you add them before this you will just confuse yourself.
The advantage of all this is that you get the basic look you want in the first stages, only adding the fine detail at the very last stages.
Example:
Progression:
