Full Color (CMYK) vs. Spot Colour (PMS) 

In the graphic design profession there are three main colour models used for design

depending on the application: CMYK, PMS and RGB

Full Colur (CMYK)

CMYK stands for cyan,magenta, yellow, and black.It is referred to as fourcolour process and is commonly used for printed materials that include images, photos or documents with many colours. These four colours are printed in tiny dots, which when layered onto each other can produce a wide range of colours.

Spot Colour (PMS)

PMS — or Pantone matching system — is the most widely used system for specifying and blending match colours. Thissystem identifies more than 700 colours, provides designers
with swatches for specific colours, and gives printers a formula for making these. They are also known as spot colours, and are used when a specific colour is required, for example. Metallic and fluorescent colours cannot be achieved by using four-colour process printing.A spot colour is necessary for printing these colours.

RGB

RGB stands for red, green, and blue. These are the colours used on a computer monitor to create colour images on the screen. However,
when it comes to printing, they must be converted to CMYK. RGB colours are used when designing for web or multimedia.
 
When to use.

CMYK colours can be used in both Digital and Offset printing and give a similar result.PMS colours can be used in Digital printing but is converted to CMYK so the results will not be as accurate as using PMS colours for Offset printing where a PMS specific ink is used to produce the colour. RGB colours can be used in both Digital & Offset printing but will be converted to CMYK in both processes. As RGB colours are used to display colour on a monitor they will vary considerably when output to Digital or Offset printers.