CMYK is an abbreviation for "Cyan Magenta Yellow Black" and is a color model that is used for offset printing when producing documents in full color. The CMYK color system subtracts the brightness of the white or light background through partial or full color layering during printing.
The CMYK printing system uses black ink that is applied over a gray color layer. This process allows the black color to become more pronounced while printing and create a bolder outline over color areas. Generally, printing done using the CMYK system sees the colors applied in the order that they are seen in the abbreviation.