Spectral Colour Model

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About the diagram

• This diagram introduces the spectral colour model. Spectral colours are the colours of the visible spectrum.
• Spectral colours are evoked by a single wavelength of light in the visible spectrum. So, every wavelength of visible light is perceived as a spectral colour by a human observer.
• The spectral colour model explains why different light sources produce different experiences of colour for an observer.

What you need to remember:

• A diagram of spectral colour is usually presented in the form of a continuous linear spectrum organised by wavelength, with red at one end and violet at the other.
• The best known spectral colours are the colours of the rainbow – red, orange, yellow, green, blue and violet.
• All spectral colours are produced by a single wavelength of light.
• The fact that we see distinct bands of colour in a rainbow, rather than a continuum of colours, is an artefact of human colour vision.
• Every spectral colour is produced by a single wavelength of visible light – the small part of the electromagnetic spectrum that our eyes are attuned to.
• Spectral colours are produced as raindrops and other transparent media refract and disperse white light causing the different wavelengths to fan out into an array of colour.
• All transparent media refract and disperse light without causing scattering.
• Spectral colour is neither an additive nor subtractive colour model because each colour is produced by a single wavelength rather than by mixing different colours.
• Sunlight produces the full range of spectral colours because at the point at which light is emitted by the sun and propagates through the vacuum of space, it contains all wavelengths of visible light.
• Light containing all the wavelengths of the visible spectrum is called white light.

Spectral and RGB colours

Spectral colour should not be confused with RGB colour:

• Spectral colours are components of the visible spectrum.
• RGB colours are produced by mixing wavelengths of light corresponding with the three additive primary colours – red, green and blue.
• A diagram of RGB colour is often represented in the form of a colour wheel and shows the colours produced by mixing adjacent colours on the wheel.

What is a colour model?

A colour model is a way of:

• Making sense of the colours we see around us in the world.
• Understanding the relationship of colours to one another.
• Understanding how to mix each type of coloured media to produce predictable results.
• Specifying colours using names, codes, notation, equations etc.
• Organising and using colours for different purposes.
• Using colours in predictable and repeatable ways.
• Working out systems and rules for mixing and using different types of colour.
• Creating colour palettes, gamuts and colour guides.

Why use colour models?

• Colour models help to relate colours to:
  • One another
  • Light sources, objects and materials
  • Experience and perception.
• Colour models make sense of the fact that coloured lights, transparent inks and opaque paints (etc.) all produce different results when mixed.
• Colour models help us manage the fact that colours mean and feel different and have different associations depending on context.
• Colours models help us manage the fact that colours behave and appear differently:
  • When emitted by different types of light source.
  • When applied to, mixed with, or projected onto different materials.
  • When used for different purposes (fabrics, electrical wiring and components, print media, movies etc.)
  • When seen or used in different situations (indoors, in sunlight, in low light, on a digital display etc.)

Additive and subtractive colour

There are two principal types of colour model, additive and subtractive. Additive colour models are used when mixing light to produce colour. Subtractive colour models are used for printing with inks and dyes. The most common colour models used by graphic designers on a day to day basis are the RGB model on their computer displays and the CMYK model for digital printing.

Remember that:

• Seeing colour results from how our eyes process light waves.
• In the real world, colours are changing all the time, appear differently in different situations and are infinitely variable.
• So colour models help to make sense of a chaotic world.

What colour models do?

A colour model helps to do any of the following:

• Decide what colours to mix to get the colour you want.
• Know what happens when you mix two or more colours together.
• Provide a name or code for a colour or a series of colours you want to use again.
• Give you a list of colours produced by a rainbow or by a digital display.
• Provide a system to mix a palette of colours from red, green and blue (RGB) or from cyan, magenta and yellow (CMY).

Spectral colour model

The spectral colour model (red, orange, yellow, green, blue, violet) is associated with rainbows and the refraction and dispersion of wavelengths of light into bands of colour.

RGB colour model

RGB (red, green, blue) is an additive colour model based on the trichromatic theory of colour vision. It is widely used in video cameras, for producing colour on digital screens and with software such as Adobe Creative Cloud.

CMY(K) colour model

CMY (cyan, magenta, yellow) is a subtractive colour model. It is the standard colour model for digital printing. Printers often include a fourth component, black ink (K), to increase the density of darker colours and blacks.

HSB colour model

HSB (hue, saturation, brightness) is a popular colour model because it is more intuitive and so easier to use when adjusting colour with digital software such as Adobe Creative Cloud.

HSB is one of a family that also includes HSV (hue, saturation, value) and HSI (hue, saturation, intensity).

Applications of colour models

Colour models have many applications including:

• Understanding colour vision.
• Mixing different coloured media eg. lights, paints, inks and dye.
• Using colour with different equipment and technologies.
• Storing and sharing colour information eg. notation systems and file types.
• Describing and naming colours in a consistent way.
• Nomenclature for describing similar things eg. systems for describing birds according to their colour.
• Comparing colours eg. swatches and samples.

Colour models, colour spaces and colour systems

• Colour models are device-dependent. This means that a colour specified as R=220, G=180, B=140 might appear differently on two digital monitors or when printed by different printers with the same specifications. In other words, the exact colour produced depends on the device that produces it not on the colour model itself.
• A colour space describes the range of colours that an observer might see. Colour spaces can be very limited when a photo is printed on a low price digital printers, large when the same image is viewed on a high definition digital displays, or huge when the original scene is viewed in bright sunlight on a summer day.
• A colour system considers all the factors that affect the observer, the colour model, how information is encoded before sending to the output device and the circumstances in which it is expected to be viewed.