Title: 4.3. RGB and CMYK spaces
Course: EN_Colour theory > Section: 4. Primary colours and colour spaces

The additive colour system (RGB: Red I Green I Blue)

= Colour mixing by red, green and blue light sources of different intensity

The RGB colour space refers to a colour representation technique in which all colours are represented as an additive mixture of the primary colours red, green and blue. It is based on the three-colour theory, a theory that describes the three primary colours red, green and blue. (https://de.wikipedia.org/wiki/Dreifarbentheorie)

It is based on the research of Hermann von Helmholtz. (https://de.wikipedia.org/wiki/Hermann_von_Helmholtz)
He had discovered that any other colour can be mixed from coloured light of three primary colours. He also applied this to the eye and assumed that there are different visual cells (the cones).

Colour in RGB space is described according to the amount of red, green and blue components.
Red component R, green component G, blue component B (RGB) = colour.
This is still the functional principle of all colour televisions and colour monitors today.

Other colours are created by additive colour mixing. The more light you add, the brighter the colour mixture becomes.

Red+Green = Yellow
Green+Blue=Cyan
Red+Blue=Magenta
Red+Green+Blue=White

Digital photography is also based on this form. Depending on the colour reproduction process, different screens are used, such as the grain screen, the line screen or the lenticular process. The technical problems of colour reproduction are mainly based on the availability of economically, technologically and technically suitable phosphors with corresponding luminescence in the required excitation range.

The description of coloured areas on web pages can be implemented by specifying the proportions of primary colours in web colours accordingly. The more colour is used, the brighter the image. If all three primary colours are used to 100 %, white is obtained.

The subtractive colour system (CMYK: cyan, magenta, yellow, black).

The subtractive model CMYK forms the basis for modern printing processes. It consists of the three colours cyan, magenta and yellow as well as the key colour black, which are printed on a (usually) white background. The more colour is applied, the darker the result. The key colour is necessary because 100 % colour application of cyan, magenta and yellow does not produce a true black, but only a dark brown.

One of the most central points in the creation of print data is the choice of the correct colour space. If data is delivered in RGB, serious colour deviations can occur. But why is it not possible to print in RGB?

In the suctractive process, the individual colours are applied to the paper one after the other, creating all possible colour shades depending on the mixing ratio. Theoretically, more than 4 billion colours are possible in the CMYK colour space. However, only far fewer can be displayed on screen and printed. In fact, the CMYK colour space is smaller than the RGB colour space.

This is where the challenge lies when creating print data. Photos, images and other elements are usually in RGB and must be converted to CMYK for printing.

Layout programs, for example, can do this.

Subtractive colour mixing with filters (0:34)