This is the coloring and shading step for Yellow Mountain by Ning Yeh from his book Landscape Lessons Book 1.
We can think of the shading or the coloring of the space as expressing Yin energy. It is expanding and testing the boundaries of the line work. It is expressive and softening the hard edges of the lines. It is providing the a focus of beauty, color and contrast of both texture and intensity. It is filling out the spine of trees and structures with life in the form of leaves and accents.