RGB colour is a model used for on-screen, like digital and web-based graphics.
Red, green, and blue light are combined in various ways to reproduce a wide range of colours on the light spectrum.
The RGB model is a light additive colour model; colours begin as black and get lighter and lighter as more colour (light) is added. This means that colours are added together to create lighter colours: in the summation of all light on the RGB spectrum, you generate white (#ffffff), and in the absence of light on the RGB spectrum, you generate black (#000000).
CMYK, also known as “process colour” or “four-colour,” is a colour model used for printing.
The acronym stands for the four ink colours used for its printing process: Cyan, Magenta, Yellow, and Key (better known as black).
The CMYK model is a subtractive colour model; colours begin as white and then get darker as more colours are combined.
CMYK has to do with ink – in the absence of ink on the CMYK spectrum, you generate white. And you generate black in the summation of all ink on the CMYK spectrum. That’s right, the opposite of RGB!