Made this edit yesterday but didn't really have time to post an explanation to what I did, so forgive me if some things have changed in your current version.
I'd suggest tuning down the black usage. Both the lightest and darkest colours should be used minimally to achieve a good balance with the shading (if you're using the troll for a game, that is, if it's going to be animated, keeping the outlines would make sense - to make him stand out).
For this reason I reduced the black usage just to vital areas where there's pretty much no light hitting that area, and made the darkest green lighter so it wouldn't be as close to the black (I also made the lightest colour lighter with a few other colours that I've changed)
Additionally I've added some reflections here and there, which again, if used for a game would probably not work very well.
There are two reasons for the abdomen's flat look:
1). the highlight
2.) the shading
The highlight implies that it's at an angle (being placed so far up next to the breast), which is amplified by the fact you didn't make gradual shading to lead up to that point, it implies that the whole green part of the abdomen is a flat surface.
note that I didn't edit the leg and several other areas, but hopefully this can still help you in some way.