I personally prefer more subtle and realisting highlighting like here: http://www.coolminiornot.com/313687?browseid=3603050 for example. The worst thing in your model is this green part of gun, you know this one which is usually transparent, it looks unnatural, so if you want to paint it my tip is, do it like this guy: http://www.coolminiornot.com/320059?browseid=3603050