In design standards, there is no specific concept of "plain color", but in color classification, plain color is actually colorless, and people are used to calling it "plain color".
Color can be divided into colored and colorless. Colorful colors have some or some color phases in the spectrum, which are collectively referred to as color tones, while colorless means no color tones. For example, black and white, and gray of each gradient formed in the process of black to white also belong to the colorless system, which is called plain color.