You did a nice job at emulating the look of official Mario artwork! Something I would suggest would be to rethink the white highlights on Waluigi. Perhaps if they were a warmer version of the colors the light is hitting it would look more like natural light, since natural light tends to be yellow.
Wario doesn't seem to have this issue and he's probably my favorite of the bunch, though they all look very nice. I did notice, though, that on Wario's left hand there is a line that ends in a rounded tip rather than tapering off like the other lines. I assume that can be fixed rather easily as they appear to be made in Illustrator or a similar vector program.