Additive & RGB colour

About additive & RGB colour

The RGB colour model used by TV, computer and phone screens involves additive colour mixing.

  • The RGB colour model produces all the colours seen by an observer on TV, computer and phone screens by creating arrays of red, green and blue pixels (picture elements) in different proportions.
  • Red, green and blue are called additive primary colours in an RGB colour model because just these three component colours alone can produce any conceivable colour if blended in the correct proportion.
  • Different colours are produced by varying the brightness of the component colours between completely off and fully on.
  • When fully saturated red, green and blue primary colours are mixed in equal amounts, they produce white.
  • A fully saturated hue is produced by a single wavelength (or narrow band of wavelengths) of light.
  • When any two fully saturated additive primary colours are mixed, they produce a secondary colour: yellow, cyan or magenta.
  • Some implementations of RGB colour models can produce millions of colours by varying the brightness of each of the three primary colours.
  • The additive RGB colour model cannot be used for mixing pigments such as paints, inks, dyes or powders.
  • The RGB colour model does not define the exact hue of the three primary colours so the choice of wavelengths for each primary colour is important if it is to be used as part of a colour-managed workflow.
  • The RGB colour model can be made device-independent by specifying a colour profile such as sRGB or Adobe RGB (1998) which ensures consistent results regardless of the device used to output an image.

Additive & subtractive colour models

About additive and subtractive colour models

There are two main types of colour models, additive and subtractive.

Additive Colour Models
  • Additive colour models are used when blending light to produce colour.
  • The primary colours for most additive models are red, green, and blue (RGB).
  • When combined at full intensity, they produce white light.
  • The additive RGB model (and HSB colour model) is central to display technologies such as computer screens, TVs and phone screens.
  • The additive spectral colour model is particularly useful for developing an understanding of the relationship between wavelengths of light within the visible spectrum and corresponding colours.
  • Additive models are based on the way human eyes perceive colour, with each colour being produced by a combination of different wavelengths. In contrast, a subtractive model is based on the way pigments reflect light.
Subtractive Colour Models
  • Subtractive colour models are used when working with pigments, inks and dyes.
  • The primary colours for most subtractive colour models are cyan, magenta, and yellow (CMY).
  • When combined cyan, magenta, and yellow produce black.
  • The subtractive CMY colour model and CMYK colour model are central to printing technologies.
  • In practice, the CMY colours often can’t produce a perfect black when mixed due to impurities in the pigments or inks, so a fourth ‘Key’ component (represented as K) is often used in printing to produce a true black.