Additive colour & the RGB colour model

About additive colour & the RGB colour model

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.

Complementary colours & the RGB colour model

About complementary colours & the RGB colour model

Red, green & blue light

About red, green & blue light
  • Because of the way the eye works, we can see all the colours of the visible spectrum by mixing red, green and blue lights at different intensities.
  • Red, green and blue are the three primary colours of the RGB colour model.
  • The RGB colour model replicates the response of light-sensitive cone cells in the retina at the back of our eyes sense colour.
  • Mixing wavelengths of light corresponding with the RGB primaries fools the eye into seeing almost any imaginable colour.

RGB & the trichromatic colour model

About RGB & the trichromatic colour model

To make sense of the physiological basis of the RGB colour model we can relate it to how the trichromatic colour model explains colour vision. Let’s look at the Trichromatic colour model first:

  • The trichromatic colour theory, which is also known as the Young-Helmholtz theory, established that there are three types of cone cells in the human eye that carry out the initial stage of colour processing that ultimately produces the world of colours we see around us.
  • Cone cells are daylight photoreceptors which means they are able to convert light into electrical charges through a process called photo-transduction.
  • The sensitivity of cone cells was established using spectroscopy which measures which wavelengths are absorbed and which are reflected.
  • The three types of cone cells were identified along with the range of wavelength they absorbed:
    • L = Long (500–700 nm)
    • M = Medium (440 – 670 nm)
    • S = Short (380 – 540 nm)
  • The trichromatic colour theory also established the visual effect of exposing a human observer to mixtures of light produced by three monochromatic light sources, one in the red, one in the green, and one in the blue part of the spectrum.
  • It proved that by incrementally adjusting the intensity of the light produced by each source an observer can be induced to see any colour within the visible spectrum.
  • The outcome was that a match was produced between how the L, M and S cone cells responded to light of different wavelengths and calibrated mixtures of wavelengths of light corresponding with R, G and B. This is the basis of the RGB colour model.
  • The fact that mixtures of red, green and blue light at different levels of intensity can be used to stimulate the L, M and S cones types to produce any human observable colour underpins almost every form of colour management in practice today.

RGB colour and colour perception

About RGB colour and colour perception
  • The human eye, and so visual perception, is tuned to the visible spectrum and so to spectral colours between red and violet.
  • RGB colour is a model used to reproduce colour in a way that matches perception.
  • An RGB colour wheel helps to simulate:
    • The effect of projecting lights with wavelengths corresponding to the three primary colours, red, green and blue onto a neutral-coloured surface.
    • The additional colours produced by mixing adjacent pairs of colours such as adjacent primary, secondary, tertiary colours etc.
  • Every imaginable colour can be produced by the RGB colour model.
  • Remember that the RGB is an additive colour model used when mixing light of different wavelengths.
  • The CMYK subtractive colour model is often used when mixing paints, dyes and pigments.