Hey, I usually stay away from CC as I am not very good at it but here are a few things I wish to suggest to you to explore more fully.
Your "black and white" image is more like a stone statue minus the stone textures. The lighting and depth is there but not enough. I can see you sort of understand basic proportions which can take anyone a while to get which is really good
but you need to tweak more. Most of the hair lacks dimension, more like it is plastic hair. I would suggestion looking at hair references and to notice how though hair is individual strands it usually falls in groups which makes it easier to manage when pixeling.
An anatomy issue I would like to put forth is these two heads are quite round. Try squaring up some of the features to give it a more humanly realistic look.
Don't be afraid to shade large chunks of the image with dark colors. It gives great illusions of depth.
Ok before I sound anymore stupid since I don't know what I am talking about I want to say good work and keep practicing.