I agree with Quo about the outline statement. I can also see that Mario's skin need a bit more saturated colors and the red and blue needs a bit more crisp, more contrast. Also, don't be afraid to break your outline as Quo said, make room for maybe additional shading that could make the sprite pop out a bit more! Below is a quick edit of what I mean, hope it helps!
(though, the little edit on the hat made mario look like luigi, welp i need some sleep!)

![[Image: oqenhk_th.png]](http://i44.tinypic.com/oqenhk_th.png)