There are only six basic colors: three primary colors red, yellow and blue, and three intermediate colors orange, green and purple, namely red, orange, yellow, green, blue and purple. Hue is the primary feature of color and the most accurate standard to distinguish different colors. In fact, any color except black, white and gray has the property of hue, and hue is composed of primary color, intermediate color and composite color.

Tone, the qualitative appearance that color can present. Different colors in nature are infinitely rich, such as purplish red, silver gray and orange yellow. Hue is the appearance title of various colors. From the optical point of view, the hue difference is caused by the length of light wavelength. Even the same color can be divided into several shades, for example, yellow can be divided into medium yellow, khaki, lemon yellow and so on. And gray can be divided into red gray, blue gray, purple gray and so on. There are six basic colors in the spectrum: red, orange, yellow, green, blue and purple, and the human eye can distinguish about 180 colors with different tones.