The difference between CMYK and RGB

The difference between CMYK and RGB

One of the many things a graphic designer needs to know is the difference between CMYK and RGB.
CMYK stands for cyan, magenta, yellow, and black. The reason black is abbreviated to K is because when plates were used the black plate was typically call the key plate because it included key information about the detail whereas RGB stands for red, green and blue.

The way to think of it is that if it’s going on the web it should be RGB and if it’s being printed it should be CMYK. There are exceptions such a spot or special colours but we don’t need to go into that today.

However not everyone knows this is the standard so here is an explanation.
Many moons ago to achieve colour on a printing press each ink (cyan, magenta, yellow, and black) had its own plate. The printer would lay down one colour, wait for it to dry, lay down another colour, wait for it to dry and so on. 

As technology has progressed digital presses have come a long way too, allowing them to print in RGB as well and your printer may accept RGB files and do the conversion for you. Whenever I prepare artwork for print I set the files to CMYK and if I’m preparing web based files I use RGB. 

Why does what I see on screen differ to what I have printed?
When viewed on screen, computer monitors give off coloured light known as RGB (red, green and blue). All colours of the spectrum can be produced by merging these three colours of light but monitors are only capable of displaying a limited gamut (range) of the visible spectrum but they do have a much larger colour gamut than printing, which is why a computer can display many more colours than can be achieved with printing and why an image or piece or artwork viewed on screen won’t always match exactly what is printed.

Venn diagram showing how CMYK and RGB colours mix
CMYK colours (on the left) subtract together to tun the page black and RGB colours )on the right) add together to create white

RGB colours are additive
Computer monitors work by additive colour mixing therefore three overlapping light sources in a vacuum add together to create white.

CMYK colours are subtractive
Printing works by subtractive colour mixing therefore if you have three passes of ink on a white substrate, by subtracting together they would turn the paper black.

Preparing files for print or web does require an understanding of the difference between CMYK and RGB.