I think you're depending too much on highlights and weak shadows to create shape on your characters. You seem to be keen on creating faces, and to that, even if stylized--You can't have both simplistic (monochromatic) and realistic (huge color variant on the face). It seems that you're shying away when doing that, the best of these three would have to be the last one, simply because you were not afraid to jump in and use both light and shadow.
About the highlights; those sharp cuts are not necessary unless you're creating some form of rim light, which are usually quite strong. Now, it is possible to somewhat mix realistic and stylized--Something that the creator of this forum, Marc, loves doing, is having a stylized character follow realistic lighting to some degree. It is why his color and lights seem so brilliant at times--And to get around his character being stylized, he uses dark and shadow, creates the shape to make it 3D and not a flat Manga-like look. If that is what you are going for, you will need to use colors and values more to get that look, you will also have to look a bit more into anatomy and maybe exaggerate some parts of the face to get that certain look you're after. I really love your last character, but looking more into it, her eyes are some weak lines. The eye might even be misplaced, and it might be too big or far off, when you consider its size compared to her lips. Either way, you're getting away with her head being tilted, as we seem to recognize things differently when they are so. So, again, her eyes are some small lines, you could do more to make them special. And I see you put some effort into forming the corner of her eyelid, but you didn't go all out. Her medial canthus is not showing, and often, there is a plane between the eye and nose that is lighter than both. I think you are doing really well, and I expect you to go really far by seeing the progress between these. However, right now, I think you need to look even more into shapes and expressing them in 3D. Sorry if this got long, but I hope it helps!
