Hege makes plenty of pixuls

[Chris2Balls [:B]]

The first sprite is my favourite, the rest suffer from banding, as Iceman404 said.
Some of the outlines aren't refined and the palettes could sometimes be smoother, more balanced and more varied ( I'm thinking of the first sprite's tail amongst others).
Most of the poses for your sprites are very similar (the legs) and lack balance.
Also, expect criticism over critique on this website
And as for the most recent sprite... you have tried to put detail in your sprite but it ended in an overkill; some parts should be lighter and others darker, the red readability issue can be solved if there are other colours to add to the mix. If you have more hue shifting you can different parts of her face and body too.
In the long term you should detach yourself from the Sonic influence, try and find a more personal way of expressing yourself.
I hope I helped in a way.