Fun as hell to paint, but hard to know what would be right or wrong. Crits are very welcome, as of now I don't know either.
You have a good idea of lighting in your b&w pieces, I think this guy looks pretty good. I find it's because he sits in the environment (in this case the white bg) quite well via shadows and glancing reflections.
Although there's not much to tell from, the colors on the latest piece looks much improved over previous color work.
Maybe study refs for the light & reflection stuff, and don't forget to practice perspective and anatomy. They can really let down a piece if not solid (I have this problem).
In facial studies, while the skulls look okay, the faces looks sorta flat ... drawing the head tilted upward or downward in frontal view is difficult to do in the beginning ... its best to start with a side view or a 3 quarter view to get used to the bone and muscle structures first.