There are two basic models used in DTP (Desk Top Publishing) software for specifying color. They are called spot color and process color. Spot color is a way of specifying that one or more pre-mixed ink colors be used on a printed piece. Each spot color is a different ink on the press. Process color is a special process used to create a wide variety of colors with only four standard inks: cyan, magenta, yellow, and black (commonly called CMYK.) Not all colors can be reproduced using process colors. Many colors such as metallic colors have no process equivalent. This is why spot colors are often added to a design even when the job requires four color process inks for color photos.
The process and spot color models arise from the two basic methods of creating colors on a printing press. When a press operator is preparing the press for a press run, he or she loads the rollers on the press with ink. Each roller assembly is loaded with a different color. As each piece of paper is fed through the press, it passes through each of the roller assemblies and is imprinted with each color in succession. Usually, one of the inks is black.
Often, one or more spot colors are added to enhance the design. If a color photo is used, then four of the roller assemblies are loaded with the colors cyan, magenta, yellow, and black. These four process ink colors are printed on top of each other in varying amounts to create all the possible process colors. Printing inks are typically semi-transparent so that overlaying colors result in a third color. For example, cyan and yellow will produce green, magenta and yellow produce red. By using spot color inks and the four basic process component inks, a press operator can reproduce designs that range from simple black only jobs to full color reproductions. Understanding how colors are put down by the presses is important for understanding how color works in your designs and how to correctly specify colors, whether spot or process, in your DTP software.
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