Very nice so far.
My three cents:
I wouldn't suggest dithering until after you've gotten everything else right, leave that work until you've gotten the piece a lot more completed.
Having the reference would be nice. I mean, the pic is already on the internet anyway, but, it's your call. It's just much harder to critique someone who is trying to follow a reference when we can't see the reference either.
I would also suggest you start shading a bit, simple lines and fills are not usually much to critique on.
About the colors.. Yeah, use only colors that you need. If two colors are different in hue, but still look similar, go ahead and use one or the other, in the other colors place. You may need more than 1 highlight and 1 shadow for certain parts though, and some, less.
EDIT: I forgot to mention, although Jonathan already said, you really do need more contrast. Skin values are too close.