Edit:
As i see it, you haven't defined the volumes of the face (using the lighting) correctly. Study the reference carefully (your own face, foto, whatever), notice how the shadows define volumes, and replicate it accordingly.
Also, the skin tones are too distant from each other, creating the need for heavy dithering (as in my example). If you want to avoid this, add intermediate colors to blend the tones, or bring them closer (i recommend the first approach, as to not induce loss of contrast as the second may, but it may add complexity in managing the increased number of colors).
P.S.: Turbo is in no way a dithering or palette expert/nerd. Take these advices at face value.
Also, it was fun playing with your face.