CMYK and RGB Colour - What's That All About?

In graphic design and print there are 2 colour models used. One is RGB and the other CMYK.

RGB
RGB is called an 'additive' because it combines varying proportions of red, green and blue to produce a variety of colours. White is the additive combination of all 3 colours.

The main purpose of the RGB colour model is to mirror how the human eye perceives colour.

CMYK
When you add red, blue or green together, secondary colours are created and in their purest combination represent the four colours of printing - cyan, magenta, yellow and black.

or alternatively

CMYK is a 'subtractive' as it removes cyan, magenta and yellow in varying degrees. A combination of all 3 colours produced black, so the complete opposite of RGB.

Why Have 2 Colour Models?
RGB colour is used for sensing, representing and displaying colour and images in televisions, image scanners, video and digital cameras, mobile phones and computer screens. It is however 'device dependant' and levels will vary from manufacturer to manufacturer or even in the same device over time.

CMYK is the model used by a printing device. Each device has a colour 'gamet' which is it's potential to generate colours. Most devices cannot replicate every colour in the spectrum.

So immediately it is clear that there is a considerable difference in the colour the human eye sees, and the colour that a printer can replicate, hence the use of 2 colour models. It would follow that if you are compiling images or designs for the web then using RGB colours would suffice, but for printing, all colours must be converted to CMYK for the printer to recognise and replicate exactly.

Worth a read - CMYK/Litho Printing