reference?

the shading should be more evident if you pay attention to the anatomy and not just the light. the clavicles, the muscles of the neck- all of the anatomy that makes up these areas has subtle effects on the lighting, so be sure to pay attention to her structure and not just fields of light and dark. also, you've made her neck really thick.

if this is for your own pleasure then ignore the following critique, but if this is for the gallery: all of the color-reduced portions will need to be reworked by hand. tracing as it is is controversial enough (plenty of pixel level work will be required in order to claim it as 'your' work), and color-reductions are banned entirely.