As the name implies, monochrome printing can only print one color, such as black, red, yellow, green, etc. (such as the inside of a book with black characters on a white background)

Two-color printing is two colors, such as black plus red (the national red header file is Add black text and red title on white background)

and 3 colors by analogy.

But four colors are different. Four-color printing can be printed in four colors: magenta, yellow, blue, and black, instead of being limited to four colors. Four-color printing is the so-called color printing. Based on this, you can also add some spot colors that cannot be called out through CMYK, such as gold, copper, fluorescent, four-color printing plus spot colors, which is 5 or even 6 colors printing.