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.

Complementary colours & the RGB colour model

About complementary colours & the RGB colour model

Human eye, light & RGB colour

About the human eye, light and RGB colour
  • Because of the way the eye works, we can see all the colours of the visible spectrum when red, green and blue lights are combined at different intensities.
  • The RGB colour model is designed to provide the exact stimuli to the light-sensitive cone cells in the retina to illicit perception of any predetermined colour.
  • Mixing wavelengths of light corresponding with the RGB primaries enables the human eye to see almost any imaginable colour including colours such as magenta that are not part of the visible spectrum.

Red, green & blue light

About red, green & blue light

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

RGB colour model in practice

About the RGB colour model in practice
  • RGB colour model works in practice by asking three questions of any colour: how red it is (R), how green it is (G), and how blue it is (B).
  • The RGB model is popular because it can easily be used to produce a comprehensive palette of 1530 vivid hues simply by adjusting the intensity of the three primaries.
  • When the saturation or brightness of a hue needs to be adjusted it is sometimes easier to switch to the HSB colour model.

RGB colour notation

About RGB colour notation

RGB colour values are expressed as decimal triplets (yellow = 255, 255, 0) or hexadecimal triplets (green = #00FF00). Computer software is programmed to recognise RGB colour values.

In both cases, the triplets determine the amount of red, green and blue used to produce a specific colour.
A decimal triplet is made up of three numbers between 0 and 255 divided by commas.
A hexadecimal triplet starts with a # sign followed by three two-digit numbers with values between  00 and FF written without spaces between.

RGB colour values are based on decimal notation (triplets with a base 10) or hexadecimal notation (triplets with a base 16).

  • Decimal notation uses 10 digits from 0 to 9 as follows, 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9.
  • The hexadecimal notation uses 16 digits from 0 to F as follows, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F.
  • Hexadecimal notation for values between 16 and 31 are as follows 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 1A, 1B, 1C, 1D, 1E and 1F.