What is spot colour?
The photons that make up light can have any frequency, but our eyes only respond to those in three narrow overlapping bands, which we call red, green and blue. This means that we can simulate any colour by shining a combination of red, green and blue light, even if the frequencies within each band aren't the same. Colour defined by three numbers, one controlling the amount of light in each band, is called RGB colour.
Printing inks don't shine, they absorb light. Cyan ink absorbs red light, magenta ink absorbs green light, yellow ink absorbs blue light, and black ink absorbs all three. This means that any RGB colour can be translated (usually by your printer driver) into a combination of inks which looks almost the same. Because the translation isn't perfect, many professional designers prefer to skip it and specify the amounts of ink directly. Colour defined by four numbers, one controlling the amount of each ink, is called CMYK or process colour. MacOS X has built-in support for CMYK colour: just use the “sliders” colour picker and select CMYK.
Sometimes even CMYK colour doesn't give enough control. When you print coloured text or simple graphics, using a single pre-mixed ink gives a less fuzzy look than a combination of four inks. Ink manufacturers produce large catalogues of pre-mixed colours, including special effects such as metallics, glitter and gloss finish. These inks must be specified by name and serial number, and are called spot colour. SpotPicker allows you to use spot colour in almost any MacOS X application.