There's some serious issues with your colours, first off you have 76 of them, you really shouldn't need more than say 16 for something like this, secondly the contrast really needs to get kicked up a good deal, you can barely see the shading on the blue parts, I just used the contrast tool in graphics gale but colours are powerful and you'll need to think about them a lot harder than I did. The shading itself isn't super descriptive, pixel art generally benefits from harder edges. Try not to hug outlines with highlights, it flattens the form, lighter colours come forward and the edge of a shape is usually the furthest away. The lineart could be a lot closer to the reference if you spent some more time on it, you've lost some of the more interesting curves. I didn't address this in my edit but black outlines inside the sprite should be avoided, they take up valuable space. You also have a good deal of banding (do a forum search). My edit is just a quick one on the arm and some of the midsection, ignore the chunks taken out elsewhere on the sprite, just a mistake of bucket filling.
Good luck.
