Cool!
I'm certainly not good enough to give any really meaningful advice, but critiquing the work of others can only be good for me too i suppose, so here goes:
They're all really nicely coloured except for the second row leftmost one. He looks cool by himself, but doesn't seem to fit with the rest of them. He seems too dark to me. All the others are sort of bright and colourful, and he's dark and grey. They might be what you were going for, and if it is then this is irrelevant, but you might want to make his beard a bit lighter and reduce the shadowing around his mouth to bring him up to the brightness of the rest.
I also don't really like the robot-looking thing bottom row third from left. Again, it might mean more to someone who's seen the full robot rather than just this section of the head, but i can only really tell its a robot because i can't see it being anything else.
Other than these two things, i think they're all great! I especially like the bottom rightmost one, he looks awesome! Love the mouth on him
