just something i'm working on..

[Vipershark]

Your biggest problem with the colors is that you're simply using too many.
I don't care if you're trying to replicate the MMX style (which frankly isn't a good style but that's another discussion entirely) and I don't care if you don't want it to look "cartoony", you're still using too many colors. Plain and simple.

This sprite alone has NINETEEN colors in it. You probably don't need half of that.
Even if you try to use the BUT MEGAMAN HAS A LOT TOO defense, it "only" has a maximum of 15 colors in it, meaning that you've overshot by four colors that you don't need.

In addition, a lot of your issues come from the fact that you're trying to cram too many colors into too little of a space.

The helmet has four shades in it and you're trying to force them all in when you don't need them.
Compare it to the megaman sprites, which have large clusters of blue pixels and generally only use two shades per area on the helmet.

Even on the chest piece which has more colors in a smaller area, it still has bigger clusters and less randomly-placed shades.
You can clearly see that Megaman's chestpiece has the large white cluster with gray nearing the edges, while yours has a tiny white highlight with pillowshading all around since you're trying to force five shades in there.

I'm going to be blunt.
Basically, your colors are dull and ugly (you need hue shifting and contrast) and you have too many of them (it has absolutely nothing to do with "16-bit or less", your sprite is flat out bad because you're shoving too much in there).

If you want I can make an edit to show you how to fix up the issues.