The four-color printing mode is a color registration mode used in color printing. It uses the principle of color mixing of three primary colors of color materials, plus black ink, and a total of four colors are mixed and superimposed to form the so-called "full-color printing". The four standard colors are: C:Cyan = cyan, also known as 'sky blue' or 'blue'; M:Magenta = magenta, also known as 'magenta'; Y:Yellow = yellow; K:blacK= Black, although some literature explains that the K here should be Key Color (positioning and registration color), but it is actually confused with the concept of positioning and registration used in plate making. The abbreviation here uses the last letter K instead of the beginning B to avoid confusion with Blue. The CMYK mode is a subtractive color mode, and the corresponding RGB mode is an additive color mode