The technique you see used here is one where only three colors are used plus the white of the paper. It is a technique that I often apply for skin tones across all color media. It gives very life-like skin where you get the impression of small capillaries and venules just under the skin. One red, one blue, and one yellow are chosen and strokes are applies in low opacity in many, many layers with occasional blending of the layers but not blended so completely that the strokes are lost. The edges are just softened.

In the first several layers, there can be a strong shift from one color to another. At one point it may swing blue and then with the addition of the yellow layer it can swing green, for example. Once enough layers go on, the shift is less noticeable.

Above - You can see the strokes are in enough to get a good sense of the color that is shaping up. This is about what it looks like just as it gets to the place it is ready for some subtle blending.

Below - This is what it looks like after some more layers and the blending has occurred. Many more layers are yet to go.