Did you do that by hand?
There's a lot of different shades, even in the outlines, and it seems kinda random.
For such a small sprite, you really don't need that many shades - you're overcomplicating what should be a simple and readable sprite.
Try using one shadow colour, and one highlight colour, for each base colour.
Anyway, I made a quick edit to his face.
As Olothontor said, the face lacks definition and readability. I added some white near his eyes, a nose, and a mouth.
Not the best of edits, but you get the picture.
