Basics of colour and printing
four-colour process printing ink
When we think of colour, it's helpful to think of spectrums. The visible spectrum contains all of the colours you can see. The RGB spectrum is comprised of the colours you can make on a monitor. The printable spectrum represents the colours we can print using a specific printing process. The printable spectrums are different for desktop colour printers, offset presses and commercial digital presses. Adding a spot colour increases the printable spectrum.
The vast majority of all products printed in a digital or commercial printing shop are printed with four-colour process similar to the four colours used in a desktop printer. All pictures, all colour art, and all colour type can be produced reasonably well with four-colour process. When there are large areas of black, printers will typically create what's called a packed (or rich) black which means that there are lower percentages of yellow, magenta and cyan under that black to help give it more strength. Inks are not as opaque as one might think!
PMS printing ink
PMS stands for Pantone Matching System. It is a system with hundreds of colours available to purchase with books that have been produced to represent those colours. There's also a book available called the Colour Bridge that shows all of the PMS colours and the closest four-colour process equivalent. Looking through the book you can see that perhaps 80% of the colours can be matched very closely with four-colour process and somewhere between 10% and 20% can't be matched closely at all.
Home Depot Orange is a great example of a colour that's hard to match: four-colour process can't get close to the PMS ink colour. Many organizations select a PMS colour as part of their corporate identity and some require that the actual PMS Ink is used when printing any of their material. This is the reason we have five and six colour printing presses: we can print four-colour process for most of the art and copy and pictures and then add a bump PMS colour for an organization's specific PMS colour. The colour bridge book is a good tool to use to determine if a PMS colour needs to be used or if it can be matched with four-colour process. Quite often when printing large areas of solid PMS colour, printing companies will apply the same colour twice in two different print units; it's called a double bump.
monitors and colour
Monitors and desktop printers are impossible to completely calibrate for accurate colour representation. Monitors live in an RGB colour space — print is CMYK. (four-colour process) — and desktop printers simply don't have the ability to reproduce tightly calibrated colour standards. We rely on expensive calibration tools to keep our proofing devices and presses in sync.