In process-color plus spot-color printing, the specific elements that use the spot color are printed using a separate plate. The other elements are separated into percentages of four colors and printed with separate press plates:
This type of printing is a combination of four-color process printing plus one or more spot colors. Process-color plus spot-color printing is the most expensive type of printing because it requires setting up one or more spot-color inks in addition to the four process-color inks. The printer may also need to make a separate press run for the spot-color ink. Process-color plus spot-color printing is used only for high-quality professional publications.
You would need to print a process-color plus spot-color job if you publication uses any colors that cannot be created in the CMYK color model. For example, you would need to add a spot color for any clear varnish or metallic and fluorescent colors. You may also need an added spot color if a or logo brand-specific artwork in the publication uses a color that can't be created using CMYK.
Every spot-color ink requires its own plate. As these are added, the cost of printing your publication will increase. It is best to limit spot colors to the minimum, even if this means settling for a color that is within the CMYK gamut because the color you want is outside the gamut.