What is Colour?

Colour is all around us. It is a pervasive part of everything we visually encounter in the world, yet it doesn't really exist.

The Oxford English Dictionary defines colour as "The property possessed by an object of producing different sensations on the eye as a result of the way it reflects or emits light." None of the individual colours we name (red, purple, orange) exist in reality - they are categories we create in order to describe and control the experience we call 'colour'.


The perceived colour of objects is a result of differences in the wavelength of light they reflect or transmit. We use the term 'Colour' to refer to the sensation caused when these wavelengths of light fall on the retina of the eye. An object will absorb some of the wavelengths of light that fall upon it. Those that are not absorbed are reflected back into the eye and these give the object its perceived colour. Human eyes register different wavelengths of light very differently. Violet has a short wavelength and high frequency, but is barely perceived by the human eye. Red has a long wavelength and low frequency and is seen very well. Red tends to dominate the sensation of other colours, which accounts for its use in traffic lights, stop signs and other warning devices.

This domination is due to the cornea being coloured yellow to prevent it from harmful ultraviolet rays. Yellow absorbs shorter wavelength colours such as green, blue and violet. With age the cornea becomes more yellow, so greens and blues become less visible.


People have been studying colour for centuries. Philosophers, physicists, chemists, inventors, mathematicians, painters and even an entomologist have all tried to create the perfect chromatic diagram. Be it a wheel, a grid, a list, a pyramid or a globe, each has attempted to capture the scientific magic of reflected wavelengths. Colour theory principles first appeared in the writings of Leone Battista Alberti (c 1435) and the notebooks of Leonardo da Vinci (1490).

Our modern understanding of light and colour begins with Sir Isaac Newton and a series of experiments published in 1672. He was the first to understand the rainbow by refracting white light with a prism, resolving it into its component colours, red, orange, yellow, green, blue and violet. Artists were fascinated by Newton's clear demonstration that light alone was responsible for colour.

His most useful idea for artists was his conceptual arrangement of colours around the circumference of a circle. This colour system was based on the primary colours of red, blue and yellow. Newton's circular diagram became the model for many colour systems of the 18th & 19th centuries. Moses Harris published a treatise on colour in about 1770 'The Natural System of Colours'. It was only 10 pages long, illustrated with engraved and hand coloured plates, but it became hugely influential. In the 18th century the RYB model (an abbreviation of red yellow blue) became the foundation of theories of colour vision, as the basis for perceiving all physical colours and equally in the physical mixture of pigments or dyes. It is still primarily used in art, craft and design education. We'll discover more about the colour wheel later on.

German and English scientists established in the late 19th century that colour perception is best described with a different set of primary colours.

Red, green and blue/violet (RGB) modelled through the additive mixture of three monochromatic lights. Additive colour works with anything that emits or radiates light. This system is the basis for all colour used on computer screens. In additive colour, white is the combination of colour, while black is the absence of colour.

Modern printing methods and technologies have brought greater understanding of how to make colour whatever the medium for paint, light, computing, and printing. Subtractive colour works on the basis of reflected light.

This system is based on the process colours of cyan, magenta and yellow. In subtractive colour white is the absence of colour, while black is the combination of colour. However, it's an imperfect system. The pigments available to use for printing don't fully absorb light. so a fourth compensating pigment called 'Key' is often added (CMYK). Without this additional pigment the closest to black we'd be able to render in print would be a muddy brown.