I remember Grievous with yellow, more serpentine, pupils; the rest is pretty close.
Ok, crap, it needs 100+ words. Your style is expressed great in this one, as always. You're missing where those "marks" (three on the left and on the right) on the sides of his head are actually part of uplifted grooves. I found a great reference picture that might help you add some more detail; like the part that joins to his mouth below the 'rivets' there.
The form, and proportion are great. All in all, I think you just need to study the image a little and add a few small details; and definitely (in my opinion) put in the "snake eyes".