I'd say using that green as a dark tone makes him look like a mossy old log or something, I made a quick palette edit but it evolved into something more.

1) palette: try to stay away from clean ramps using the same hue and sat, start with something like a blue or red and work it through the color you want in to a yellow or something.
2) I shittily shaded the head; but it seemed like everything except the top of the head was shaded from the top-left. I also changed around the shading of the chest and a few other areas to try and make it fit better.
3) I'm sure there's a reason why you weren't dithering, but it can help you out in some spots especially when blending 2 different hues together.
4) alot of your lines seemed to be padded and AA'd according to outlines, ie. top of the head, bottom of the jaw, other outer-surface areas like the shoulder, and part of the neck, try to stay away from that, it makes things seem too thick at parts and other not-good things.
5) if you're using most of the colors everywhere, make sure you actually do that. you use the gree to AA the dark skin to the darkest hue in almost every area except the neck, it gives a sloppy and unfinished feeling.