Color separation is the process by which original full-color digital files are separated into individual color components for four-color process printing. Every element in the file is printed in a combination of four colors: cyan, magenta, yellow, and black, known as CMYK in the world of commercial printing.
By combining these four ink colors a wide spectrum of colors can be produced on the printed page.
In the four-color printing process, each of the four color separations is applied to a separate printing plate and placed on one cylinder of a printing press. As sheets of paper run through the printing press, each plate transfers an image in one of the four colors to the paper. The colors—which are applied as minuscule dots—combine to produce a full-color image.
The actual work of making the color separations is usually handled by the commercial printing company, which uses proprietary software to separate your digital files into the four CMYK colors and to transfer the color-separated information to plates or directly to digital presses.
Most print designers work in the CMYK model to more accurately predict the appearance of the colors in the final printed product.
CMYK is not the best color model for documents destined to be viewed onscreen.
They are best built using the RGB (red, green, blue) color model. The RGB model contains more color possibilities than the CMYK model because the human eye can see more colors than ink on paper can duplicate.
If you use RGB in your design files and send the files to a commercial printer, they are still color separated into the four CMYK colors for print.
However, in the process of converting the colors from RGB to CMYK, there can be color shifts from what you see onscreen to what is reproducible on paper.
Graphic designers should set up their digital files destined for four-color separation in the CMYK mode to avoid unpleasant color surprises. All the high-end software—Adobe Photoshop, Illustrator and InDesign, Corel Draw, QuarkXPress and many more programs—offer this capability. It is just a matter of changing a preference.
Exception: If your printed project contains a spot color, a color that usually must match a specific color exactly, that color should not be marked as a CMYK color. It should be left as a spot color so when the color separations are made, it will appear on its own separation and be printed in its own special color ink.