An excercise in spriting

[Vipershark]

You have way too little contrast between your shades, man.
On the face and hair in particular (also because they're mostly the same between both versions of the sprite) I honestly can't see your highlights or shades on the 1px scale versions at all and instead your colors just look like blobs of orange and skin with some outlines.

I didn't check all of your colors, but if I remember correctly the hue on the hair only goes up/down three degrees and the hue on the skin didn't get changed between shades at all. All of your colors need hue shifting and more contrast, and not just the skin and hair.
As they are they just all blend together.