Why do sketches have to be in tune?

Sketch is realistic, any object has different faces, each face has different light intensity, and the front face is also different, so there are shades. Sketch is to express these colors, and to express these colors is to arrange colors, which is a necessary step in sketch.

First of all, the arrangement of tones is to observe the relationship between depth, count tones, output the first, second and third tones in this order, and then start to arrange, paying attention to the difference between depth and tone.

In fact, the most important thing is to observe this step, come on, accumulate more experience and gradually understand.