The third lightest green- the turquoise colour- could be a little darker. Likewise, the second darkest green could be a little lighter as it seems a bit difficult to see from the black. As it is rigt now, that second darkest black feels like more of an extension of the border than a separate colour, and it looks a little harsh and ugly in places like the leg and foot on our left.
Not too sure about the highlights on the gun, mostly because the way you've used outlines on the rest of hte sprite makes me want to see the gun have outlines. But I see why you've got them, so it might be a good idea to keep them.
Other than that, I really can't pick out much here I don't like. I like the pose, I think you made some reall nice use of your space. If you were to stick it in some game right now, it would fill out the collision rectangle real well, which is usually on my mind a lot when I make sprites.
I'd really like to see this animated, so get to it!