About Colour Modes
When working in Photoshop or Illustrator, you have the option to set your document’s colour mode to CMYK or RGB. There are some other colour modes, but CMYK and RGB are the two you really need to know about.
CMYK is the standard colour mode for sending documents – whether it’s a magazine, newspaper, flyer, brochure, annual report and so on – to the printer. It stands for cyan, magenta, yellow and key.
‘Key’ in this instance means black. It’s referred to as key because in four-colour printing, cyan, magenta and yellow printing plates are carefully keyed, or aligned, with the key of the black key plate.
When you send a job to the press, cyan, magenta, yellow and black plates are made (on a traditional press, anyhow) and then aligned to print on paper. You can add Pantone, or fifth colours, as separate plates.
RGB stands for red, green, blue, and is used for screen output. Because CMYK has a more limited colour gamut than RGB (which is essentially what the eye sees and how screens output) you can experience a loss of colour when converting from RGB to CMYK in these applications.