There are many types of colors on a color artwork or color photo. It is almost impossible to print all colors in one color. The printing method used is four-color printing. Firstly, the original manuscript is decomposed into four colors: blue (C), red (M), yellow (Y), and black (K). Then, during printing, each color is synthesized. The so-called "color separation" is based on the principle of subtractive color, using the selective absorption characteristics of red, green, and blue color filters for different wavelengths of color light, and decomposing the original manuscript into the three primary colors of yellow, magenta, and cyan. In the process of color separation, the color light absorbed by the color filter is the complementary color light of the color filter itself, so that on the photosensitive film, a negative film of black and white images is formed, and then a mesh is added to form a dot negative film, and finally copied and dried into various color printing plates. This is the earliest principle of photographic color separation. Due to the development of printing technology, we can now use pre press scanning equipment to color separate, sample, and convert the original color into digital information. That is, using the same method as photographic plate making, the original color is decomposed into three colors: red (R), green (G), and blue (B), and digitized. Then, the computer uses mathematical calculations to decompose the digital information into four colors: cyan (C), red (M), yellow (Y), and black (K).