More good progress, I gotta say!
At a glance the fish-bot almost seems pillowshaded. Looking closer it does follow a clear lightsource, but if you get rid of the lighter colors it becomes evident there's a band of shadow around places where the light hits. The head of the sprite is also a problem. Try shading it like a sphere.
Lower right design seems more like a robot than the lower left, probably because of the grays. I would go for a cross between the two, metal legs/green mouth.
Metal is shiny and has pretty high values, so I added in a lighter color to reflect that, missed out on the basic reflections here, but you get the idea. This could also be applied to BigDig (Note there are speculars on the fishbot as well. Used the lightest metal color for that.)
Quick edit here to demonstrate what didn't get through the above text.
HTH.
May edit this post later.