Mixing light & pigments

As explained above, trichromatic processing of light entering the eye involves three types of cone receptors in the retina working in concert with bipolar cells to carry out an initial stage of colour processing.

One of the more important implications of trichromacy is that it is possible to match almost any naturally perceived colour by projecting wavelengths of light corresponding with red, green and blue onto a screen or surface so that the resulting admixture is reflected into the eyes.

We are exposed to mixtures of wavelengths of light corresponding with red, green and blue whenever we look at a phone or computer or any other type of digital screen. As the intensities of each of these three wavelengths are increased or decreased we see different colours. It is an astonishing fact that simply by manipulating different combinations, some digital screens are able to display a total of 16.78 million colours by combining 256 settings for red with 256 for green x 256 for blue.

A similar process takes place when we look at magazines or other forms of digital printing. In this case, cyan, magenta and yellow inks are used in place of light and different colours are observed as the amount of each pigment is adjusted. Digital printing also uses at least one additional black ink to add tonal depth to printed colours. Digital printing usually involves applying tiny dots of colour onto white paper so that the colours overlay one another and partially or entirely prevent incident light from being reflected off the surface and into our eyes.

There is an important difference between using red, green and blue light sources to produce colour on digital screens and using cyan, magenta and yellow inks for printing. When RGB light sources are added together in different proportions they cause our eyes to produce the impression of all the colours in the visible spectrum. When inks overlay one another they subtract from the range of possible wavelengths that can reflect off the paper. When cyan and magenta are overlayed they reduce the amount of blue light that reaches our eyes, when cyan and yellow are overlayed they reduce the amount of green light that reaches our eyes, and when magenta and yellow are overlayed they reduce the amount of red that reaches our eyes. When CMY are overprinted they prevent red, green and blue from reaching our eyes.

A human observer with trichromatic vision exposed to a mixture of the three wavelengths of light corresponding with red, green and blue can see a full range of colours if they are combined in the correct proportions. Red, green and blue, when used in this way, are called additive primary colours. This is the basis of the RGB colour model.
A human observer with trichromatic vision exposed to a mixture of three pigments corresponding with cyan, magenta and yellow can see a full range of colours if they are mixed together in the correct proportions. Cyan, magenta and yellow, when used in this way, are called subtractive primary colours. This is the basis of the CMY colour model.
All vision-related technologies take advantage of the trichromatic nature of colour vision and of the remarkable discoveries that are associated with it.
The screens of televisions, computers, mobile phones and digital projectors all mix wavelengths of light corresponding with red, green and blue (RGB), to produce the richly coloured images that we take so much for granted.
Digital printing meanwhile uses cyan, magenta and yellow inks (CMY) inks but also adds black (CMYK) to produce the appearance of deeper shadows on the printed page.
Additive colour mixing is not only applied to the output of colour information to digital screens and paper. Sensors in cameras and scanners work in a similar way, encoding and recording inputs on red, green and blue channels.

The terms trichromacy and trichromatic processing are used to describe the biological operations taking place within our eyes. When this underpinning of colour vision is exploited to intentionally match colours we want to mix and display it is often referred to as a tristimulus system of colour matching. Clearly, both the RGB and CMY colour models are tristimulus systems.

A tristimulus system visually matchs a colour perceived under standardized conditions against mixtures of the three primary colours belonging to a chosen colour model.
Each colour model has methods of notation for keeping track of the exact amounts of the primary colours needed to match a perceived colour.