Part of the issue here appears to be that you are shading every part independently rather than shading the whole image, which leads me to believe that you do not have the volumes involved firmly in mind. It might not be a bad idea to break this image down into three-dimensional shapes and do a 4-value gray scale study of just the volumes, then transfer what you find there back onto the image you are working on now, and then start coloring from there.
Overall i think the head-on pose that you have chosen is working against you because the volumes are hard to make out. I, for one, am not familiar with the character and so have no idea what the volumes might be, and have a hard time judging from your image.
