This bit
Alright!
First, I personally liked the colour white better, but the brown does look cleaner. I'd make it white and then clean up some of the darker lines in it.
Kinda all over the place you need to make sure to blend a bit better, but plenty of other people have td you that so I won't dwell.
You were trying to make the cheeks look gaunt, right? Based on the location of the shading difference that was my first guess at what effect you were going for. I think the shading on that is much too dark, though. I'd honestly have to see it on the sphere to make better judgement, but looking at the flat it seems a bit extreme. No need to blend this area, but do make it look less out of place and lighten up on the shading.
The black lines on the... oh.. is there a word for that bit just above the mouth? Anywho, the black lines should be cleaned up and I think you should be able to emphasize this feature without the harsh black.
Looking at the hair on the flat, it looks like on the back of the head the hair won't look great direction/placement wise. Like, the hair would be fanning out in either direction and the very back would be higher than the bits around it. I could be wrong, but make sure to touch up on that if I'm not...