You're definitely gonna need to tweak the colors if you want to get the subtlety, adding a new one is probably the easiest way, without destroying the contrast of the whole thing. That color could furthermore be used as an AA shade.
Just a quickie showing what you might be able to do with it.
Though it could very well kill what you were trying to do. Also I tried to get rid of the banding that was going on on the lower face area.
EDIT: Yeah, what Lawrence said.
EDIT AGAIN: She doesn't look like she's smiling in mine at all, because of that dastardly last second addition of the shading on the face. Yeah.