Spot Colour is used by actually mixing ink to the desired colour rather than using the CMYK process to achieve it. The printer will mix varying amounts of colour to reach the correct consistency and then this is printed directly onto the document.
Its not un-common to see a â€˜full colourâ€™ print job having an additional spot colour or â€˜specialâ€™ as its known added to it either during the actual printing process or added after in a seperate process. Some large printing machines have an additional facility for this, taking form a 4 colour machine to a 5th or 6th colour machine, so they are able to print the whole document using full colour and two spot colours in one pass.
Spot colour is common when metallic colours are required, whereby they will not be achievable using the CMYK process.