To reproduce full-color photographic images, typical printing presses (and some inkjet printers) use 4 colors of ink. The four inks are placed on the paper in layers of dots that combine to create the illusion of many more colors. CMYK refers to the 4 ink colors used by the printing press — the the subtractive primaries plus black.
- C is cyan (a blue-green color)
- M is magenta (a reddish pink color)
- Y is yellow
- K is a pure black ink, the key plate or keyline color
CMYK is not the only full color printing process but is the most common. Other full color methods include 6C Hexachrome and 8C Dark/Light (6 colors and 8 colors instead of just 4).
There are differences between images in RGB (how most digital images start out) and CMYK so that when converting from RGB to CMYK there may be some shifts in the color. Companies that have precise corporate or logo colors may find that when converting their print logo for use on the Web (or the other way around), the logo colors may not look exactly the same. When CMYK colors are displayed on screen, such as on the Web or in your graphics software, they are just approximations of what the color will look like when printed. There will be differences.
A mistake often made when submitting artwork for 4-color printing is not converting the images to the CMYK color space. This is needed so that the file can be separated into the four colors (see illustration) so that a separate printing plate can be made for each of the colors.