The skin on your color studies is pretty flat, but that's easy to fix. The color you're using now is good for the lit surfaces, but the shadows should be cooler, like a purple, and the middle range where the light turns to dark should be a little more red. That'll add what 3D artists call subsurface scattering.
A very clear example is, on Emma Stone, the shadow under her chin could be a much cooler purple with a slightly red-tinted border.
Another tip for painting I've recently discovered that will really speed things up is to start with hard edges, then blur if you want them softer. I would always start with the airbrush and keep repainting the same area to eventually arrive at the shadow I wanted. It you start by painting in your colors and shadows boldly, you'll save a lot of time.
A lot of shadows need harder edges than you might think at first, too, so it helps give more of a range to the painting.