To ensure that a printer uses exactly the color that a designer intends, color systems were developed. The most common of the spot color standards is the Pantone Matching System or, PMS as it is more commonly known. In addition, designers will want to have a set of swatch books that show printed examples of the colors and their codes.
Swatches are a more reliable method of matching or choosing colors than doing so on-screen.
This is because monitors are illuminated by light behind them which makes colors seem brighter on-screen than they are when printed on paper. Though some may view spot color printing as being much more limited than CMYK printing, there are many interesting possibilities. Two or more spot colors can be mixed to create interesting colors and effects. Spot colors can be combined to create duotones, tritones and quadtones that can be very effective and can add some visual interest to otherwise grayscale images. Unfortunately, it's difficult to proof spot color jobs.