I think two-colour shading's fine for now, myself. You (Rosier) still have plenty you can still learn without complicating things. For example, the contrast levels are rather low, and using two colours is a great way to build confidence with that. If you can get to the point where you can confidently use only a light and a dark colour to shade, adding more colours for a smoother transition will feel very easy, and you'll be able to focus on perfecting pixel-level stuff instead of juggling that with learning colour.
In addition, you're still shading a bit like pillow-shading in many cases, and even when you're not, there isn't a strong sense of light direction, and I'm not sure the forms you're conveying with the shadows are what you mean. For example, the first girl's shape suggests she has a thin waist and a flat stomach, but the shadows suggest a round belly, and on the latest one, your shadows suggest all kinds of dips on her thighs.
I know anatomy's been your focus, and I guess the fact that my critique all has to do with your shadows means you've been getting better at that (assuming you're learning from DesignDoll and not just copying what it spits out, anyway xP).