When two RGB colors are mixed equally they produce the colors of the CMYK model, known as subtractive primaries. Green and blue creates cyan (C), red and blue creates magenta (M), and red and green creates yellow (Y). Black is added to the model because it cannot be created with the 3 subtractive primaries (when combined they create a dark brown). The K, or “key,” stands for black.
CMYK in the Printing Process
The four-color printing process uses four printing plates; one for cyan, one for magenta, one for yellow and one for black. When the colors are combined on paper (they are actually printed as small dots), the human eye sees the final image.
CMYK in Graphic Design
Graphic designers have to deal with the issue of seeing their work on screen in RGB, although their final printed piece will be in CMYK. Digital files should be converted to CMYK before sending to printers, unless otherwise specified. Because of this issue, it is important to use “swatches” when designing if exact color matching is important. Swatches provide a designer and client with a printed example of what a color will look like on paper. A selected swatch color can then be chosen in Photoshop (or a similar program) to insure the desired results. Even though the on-screen color won’t exactly match the swatch, you know what your final color will look like. You can also get a “proof” from a printer, which is an example of your printed piece provided before the entire job is run.