The CMYK model, or four color processes, is one of the most common models currently used in color printing. CMYK stands for cyan, magenta, yellow and black. Newspapers, magazines and various other products are printed using CMYK.
The CMYK image mode in Adobe Photoshop refers to the four-color CMYK printing process, in which cyan, magenta, yellow and black inks are mixed to generate a range of colors. Images intended for print should always be converted to CMYK prior to printing.
CMYK is a subtractive color model, meaning that it works by adding color to a white background, thus reducing (or subtracting from) the brightness of the background. Black ink is included in the process because it is difficult to mix black from cyan, magenta and yellow.
Colors in CMYK are separated into the percentage of each ink needed to generate them. For example, green might be created by mixing 100 percent cyan, 0 percent magenta, 100 percent yellow and 0 percent black.
Converting an image to CMYK changes its channels to cyan, magenta, yellow and black. There is no perfect system for converting from RGB (red, green, blue) to CMYK. However, Photoshop's CMYK setup option does offer some help in the process.
Since computer displays are RGB, they do not accurately represent how CMYK images will look when printed.