While the calligraphic body of a painting is mainly used to create power, movement and body, the ink shading and colour are to establish space and volume.
A simple example we see here. The reed grass is drawn with fine brushwork, but to fill the space between the bushels we see broad strokes of light ink. They connect the patches in the mind to the surface of a lake.
In this case we can see how ink or colour shade is applied.