RGB colour model

RGB colour model, a structured system used in digital devices and light-based media to create a gamut of colours from a small set of primary colours—in this case, red, green, and blue (the name of the colour model comes from the first letter of each primary colour’s name). It is one of the three most common colour models, which include CMYK (cyan, magenta, yellow, key [black]), primarily used for colour printing, and RYB (red, yellow, blue), often used in the visual arts.

The RGB colour model is considered an additive system, because it adds wavelengths of the primary colours red, green, and blue together to create a broad range of colours. The process can be demonstrated by using three light projectors, each one fitted with a coloured filter so that one projects a beam of red light onto a white wall, another a beam of green light, and the third a beam of blue light. If the red and green beams were to overlap on the wall, they would create yellow. If the intensity of the green light were reduced or the saturation of the red increased, the light on the wall would become orange. If all three lights were combined, they would create white. This additive process differs from the subtractive process, one of which is the RYB colour model. The RYB colour model is used by artists primarily working in paint. If all of its primary colours—red, yellow, and blue—are combined, theoretically they would create black. This is because the pigments of paint selectively absorb and reflect light to create colour. For example, a yellow pigment absorbs blue and violet wavelengths while reflecting yellow, green, and red wavelengths. If yellow and blue pigments are mixed, green will be produced, since it is the only wavelength that is not strongly absorbed by either pigment.

Computer monitors, colour televisions, and similar devices use the additive process to create a variety of colours on screens. A magnified image of a screen reveals that the colours are formed in much the same way as in the above example using the three projectors with coloured filters. Each pixel on a screen comprises three small dots of phosphors, one of which emits red light when activated by an electron beam, another green, and a third blue. If the screen displays a patch of yellow, for example, the red and green phosphors in that patch of pixels are stimulated while the blue phosphors in the pixels are not.

The basis for the RGB colour model comes from English physicist and mathematician Isaac Newton, specifically his series of experiments with light in 1665 and 1666. In one of his famous tests, Newton held up a glass prism to a ray of light as it entered a darkened room. He later documented his findings in Opticks (1704), describing how the white light split into red, orange, yellow, green, blue, indigo, and violet light. He concluded that white light is a combination of all colours, and he became the first person to hint at how colour is perceived by humans.

The mixing of coloured light was furthered by English physicist Thomas Young and German physicist Hermann von Helmholtz in the trichromatic theory of colour vision (also called the Young-Helmholtz theory). In the first years of the 19th century, Young definitively established the wave nature of light and then calculated the approximate wavelengths of the seven colours recognized by Newton. He went on to hypothesize that the human eye perceives colour through three photoreceptors (later called cones), which are sensitive to specific wavelengths on the visible spectrum, and that humans could see a broad array of colours through internal combination. Young’s theories were greeted with skepticism, and eventually he moved onto a different project—helping to translate the recently discovered Rosetta Stone. In the mid-century his theory was taken up by Helmholtz, who postulated that each of the three receptors in the eye could receive only certain wavelengths: one could detect only short wavelengths, another only medium wavelengths, and the third only long wavelengths. He went on to argue that if all three of the receptors were stimulated at the same time with equal amounts of intensity, the eye would perceive white. If the intensity of one wave were decreased, however, the perceived colour would change.

While Young and Helmholtz proposed that colour vision was based on three colours, neither established what those three colours were. About the same time as Helmholtz was forming his theory, however, Scottish mathematician and physicist James Clerk Maxwell was experimenting with colour vision. By using coloured spinning tops of his own design, he demonstrated that—in opposition to the primary colours red, yellow, and blue used by artists—the colours red, green, and blue could produce a broader range. Maxwell later showed that he could create a full colour photograph by using red, green, and blue filters over a camera lens. He had British photographer Thomas Sutton take three black-and-white photographs of a Scottish tartan ribbon tied in a rosette, each time with a different coloured filter. They then printed the photographs on glass and projected them simultaneously on a wall during a lecture in 1861. This projection has often been called the first colour photograph, and indeed Maxwell’s three-colour system provided the foundation for modern photography. The projection was also the first demonstration of the RGB colour model. 

Over time, the different wavelengths described by Helmholtz were recognized as being associated with red (long), green (medium), and blue (short). Although the trichromatic colour vision theory is now thought to be only one part of a complex process of human vision, it demonstrates that the RGB colour model most closely resembles eyesight and is thus considered one of the more-accurate colour models.