You can use custom inks, called spot colors, in addition to, or in place of, process colors. For example, instead of using the four process colors to reproduce artwork consisting of black text and bluish-green line drawings, you could use two spot colors—one of black, and one representing the exact shade of green. You can also use spot color inks to produce colors not reproducible by CMYK inks, such as varnishes or fluorescent and metallic colors. In addition, you can mix two or more spot colors together or mix spot colors with process colors to create mixed inks.

You can color artwork with process colors, spot colors, or a combination of both. When printing separations, you can convert spot colors to their process color equivalents so that they will be printed on the CMYK plates.