When I had enough of it I did raise the contrast a bit to make the different tones better visible.

In the next step I used the smudge tool to soften the edges and remove the cell shading effect.

When closing the layer with the linework it looked like this. (°__°)

In the next step I used a soft airbrush tool to give the shadows more depth and correct some rough looking spots.You can see the steps on the images below.

After I finished this step I made the layer with the line work invisible because I don’t need it for the next steps.
With a fine brush I repaint the eyes and other details witch are now missing with the invisible line work and add additional fine shadows.

After finishing the rough shading I did adjust the colors because they look very unnatural since I only used 1 color. In the next step I did fine color and shadow tuning,. With a soft brush I did paint over the face with different blue and green colors on a new layer with a multiply setting. Then I merged both layers to a single one.

Having plain black color in my image looks unnatural so I over painted the black areas using a very soft and fine brush with a low transparency. I used the tones displayed on the image next to the eye.

Also I did correct the nose and some other fine shadows I considered as to dark or to hard (right).

 

In the nest step I did raise the contrast a little bit and added some more soft dark shadows.