Gouache is used in the exam because its composition, characteristics and color are between watercolor and oil painting pigments, which is convenient to use and can reflect the effects of watercolor and oil painting.

The pigment color of traditional Chinese painting is unique, which is not suitable for examination painting other than traditional Chinese painting. Oil painting is not suitable for college entrance examination because of its complex use.

Gouache and oil painting pigments are generally light-colored and covered with dark colors, which is not absolute. As a painting, there is no law that stipulates who will cover it in order to achieve a certain effect.

The depth of a stroke is a question of painting skills, and gouache pigments and oil painting pigments can be used.