I like the proportions. It would be nice to see these animated. Erstus is right about shifting the hue. In some ways, it seems counterintutive the change to color like that, but it makes a big difference.
Nice and compact sprite. Color ramps would be better if you made them tilted in hue, for example - pink to purple. 
I would rather see you submit an animated piece than a sprite sheet. Also, this guy could maybe use more contrast. He's cute, though.
https://www.dropbox.com/s/0z15nv41wgdwzr2/robo.exe