A little Mario thingy and some more!

[Virtuaboy123]

For the mario, I'd start by not having so many colors due to its size, but if you decide too use alot of colors use better variation/contrast.
The whites and yellows are way to close in terms of color.

Shading helps too, here's how I'd approach it.

Still not very good, but I minimized/shared the amount of colors Mario uses for his skin and red clothing. Again also take the fact the sprite is small so too many colors just leads to wasted space, especially if they are like one peg lighter than the next.