In offset printing, a spot color is any colour generated by an ink (pure or mixed) that is printed using a single run. The term spot colour to most technicians mean any colour generated by a non-standard offset ink; such as metallic, fluorescent, spot varnish, or custom hand-mixed inks.
When making a multi-color print with a spot colour process, every spot color needs its own lithographic film. All the areas of the same spot color are printed using the same film, hence, using the same lithographic plate. The dot gain, hence the screen angle and line frequency, of a spot colour vary according to its intended purpose. Spot lamination and UV coatings are sometimes referred to as 'spot colours , as they share the characteristics of requiring a separate lithographic film and print run.
There are various methods to incorporate rather sophisticated patterns of spot colours in the final prepress artwork. Software applications such as Adobe InDesign and Adobe Illustrator generate spot colours as additional channels. Adobe Photoshop can also be used to generate soft edges (widely known as feathered edges) of spot colours The dissolve effect provided by Adobe Photoshop layer patterns can be generated for any spot colour.
Generally the cost and potential for problems for a print job increase as one adds more spot colors, due to the increased cost and complexity of added process inks and films, and requiring more runs per finished print. However, because of the complicated process, spot colours are effective at preventing forgeries of money, passports, bonds and other important documents. Money printing for example, uses secret formulae of spot colors, some of which can be seen by the naked eye and some that can only be seen by using special lights or applying certain chemicals.
There are different brands of spot color inks. The dominant spot colour printing system is PANTONE. The Pantone Matching System or PMS consists of over 1,000 colors of ink. Other spot color systems include TOYO, DIC, and ANPA. Spot colour classification has led to thousands of discrete colours being given unique names or numbers. There are several industry standards in the classification of spot colour systems, such as:
- Pantone, the dominant spot color printing system in the United States and Europe.
- Toyo, a common spot color system in Japan.
- DIC, another common Japanese spot color system.
- ANPA, a palette of 300 colors specified by the American Newspaper Publishers Association for spot color usage in newspapers.
- GCMI, a standard for color used in package printing developed by the Glass Packaging Institute (formerly known as the Glass Container Manufacturers Institute, hence the abbreviation).
- HKS is a color system which contains 120 spot colors and 3250 tones for coated and uncoated paper. HKS is an abbreviation of three German color manufacturers: Hostmann-Steinberg Druckfarben, Kast + Ehinger Druckfarben and H. Schmincke & Co.
- RAL (color space system) is a color matching system used in Europe. The so-called RAL CLASSIC system is mainly used for varnish and powder coating.