What are spot colours?
Colour is created on printed material by mixing inks. In most cases, four inks are used: Cyan, Magenta, Yellow, and Black (CMYK for short). However, when printing many copies of the same artwork, some areas of flat colour (that is, areas filled with the same colour) may not appear consistent. This is due to many factors, such as ink density, temperature, paper quality, etc. If you select a colour in CMYK, it may also print as something entirely different, depending on which printer prints it.
So, in cases where a designer wants to ensure that a colour will print exactly the same on many copies (for example, a corporate colour on stationary or packaging), she can specify a spot colour instead.
Spot colours are special colours that are guaranteed to look the same, no matter who prints them, no matter how many times you print them. There are a few spot colour systems out there, but the industry standard is the Pantone Matching System (or PMS for short).
Designers
can select colours from swatch books that contain samples of all the spot
colours available. They can then be assured that the colours will print
just as they expect them to. You can order these swatch books from Pantone.
Spot colours are also useful when you're on a limited budget. Instead of paying for four-colour printing (CMYK), you can design your artwork using one or two spot colours and get the job done at a lower cost, while simultaneously providing consistent colour every time.
Keep in mind that an ink will look different if it's printed on a matte paper stock vs. a glossy, coated paper stock. For this reason, there are different PMS swatch books available, in Coated, Uncoated, as well as a few other formats, such as mettalic colours.
One important piece of advice: Never match a spot colour by its appearance on a monitor. Always select a colour from a swatch book. Computer monitors display colour by shining light from behind the screen, while printed colour is illuminated by reflected light... they're very different.
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