For outlines, create a thin (art) brush and use no fill and black stroke.

For shadows, use no stroke and a black fill and turn down the opacity under the appearance tab while drawing.

For doing the skin base.. use grey scale with k value at  40% – no stroke with fill – it’s own layer

For  doing medium contours ( slightly darker areas on skin but not the shadows ) use use grey scale with k value at  60% – no stroke with fill – it’s own layer

For  doing medium contours ( slightly darker areas on skin but not the shadows ) use use grey scale with k value at  75% – no stroke with fill – it’s own layer

Any new color should be on it’s own layers