RGB is the three primary colors, red, green and blue. The color used for printing is CMYK (cyan, red, yellow and black), and printing is printed by the percentage of these four colors. Color can be divided into chromatic light and pigment. Chromatic light is light and pigment is pigment. Color is the inherent property of an object. Color is the inherent spectral expression of an object. Chromatic light is divided into visible light and invisible light. Visible light such as red, orange, yellow, green, blue, purple and white. Invisible light such as infrared, ultraviolet γ Ray, radio wave length, etc. the color presented by an object is affected by ambient light and human physiological characteristics, but its essence is unchanged. The three elements of color are hue, lightness and saturation. Hue is the characteristic of color, lightness is brightness, saturation is purity, and the three elements of color can be represented by symmetrical cones (please refer to chromatics) , the lightest and darkest colors are less obvious. The three primary colors of color light are red (R), green (g) and blue (b) (used in Photoshop, TV, etc.), and the three primary colors of color material are green (c), red (m) and yellow (Y) The mixing of the three primary colors of color and light is the principle of additive method, and the mixing of the three primary colors of color material is the principle of subtractive method; the brighter the mixing of the three primary colors of color and light is, for example, the same amount of RGB phase is mixed into white, and the darker the mixing of the three primary colors of color material is, for example, the same amount of CMY phase is superimposed into black. No matter the superposition of the three primary colors of color light and color material, other colors can be mixed, but the range of color light is far greater than that of color The range of materials, that is, the display range of RGB is greater than that of CMY, which is why the image of the display looks better than the color of the printed matter. Color printing is to turn a color original into a color print according to the principle of three primary colors of color materials, but it is usually completed by four colors: blue, red, yellow and black. Adding black is because it is too difficult to print black and gray (i.e. neutral gray) due to the purity and accurate amount of color. Therefore, black is added. Color printing in printing refers to four-color printing, i.e. blue, red, yellow and black. Printing can be divided into four-color, monochrome, two-color, spot color and other printing.