Very well done!
I think you did a tremendous job on colours and cluster placement. One thing that could be improved though is the size of the eye sockets,they look a little bit too small.
Agreeing with everyone else, top notch colours! Really great overall pixelling.
I really like this, very lovely choice of colours. Very clean pixel clusters too!
I'm a sucker for well-used bright colors, so this is, of course, very appealing to me.
This skull looks quite.. juicy? 
Anyways, the rendering is slick and excellent. The colors are great. You're getting quite good.
This skull is viewed straight on from the front, as well as a lot of your other works are. I'd challenge you to start choosing more interesting and complex angles for your drawings, to challenge yourself, if you really want to keep improving.
Really, really lovely colours. Those spots of highlight work amazingly well.
Thanks for the comments guys!
@Skeddles - No haha. I like to make skulls a lot and always end up putting an "M" on the top.
@Mandril - Thank you! I have to give credit to Storm for the colors. It was originally for a challenge. Also I agree about the eye socket. I was waiting for a second opinon on that hah.
@Dr D - Thanks a lot :) Good Point. I'll try to expand on my perspectives a lot more.