Colors you intend to print commercially are typically mixed with four inks: cyan, magenta, yellow and black (CMYK). If your QuarkXPress or InDesign document contains just a few colors (three or less), consider using spot colors. If you use spot colors, only the specific inks you need will be added to the press when your file is printed—and you will only be charged for those inks. If your layout uses four colors or more, use process colors. In this scenario, the individual CMYK inks are added to the press, one at a time—and you are charged for four inks. If your layout has a photograph, the image likely contains thousands of colors. However, since the colors are mixed with components of CMYK, you will only be charged for four inks, not thousands.
A typical rule of thumb follows: if you use three colors or fewer in your layout, use spot colors. If you use four colors or more, use process colors. It is possible to combine the use of spot and process colors in the same layout. Although expensive to do so, the practice can create some very nice results. For instance, you might create a layout with photographs (which would need CMYK process inks), plus a logo (the logo might need just one or two spot colors).