# Spectral Colour & the RGB Colour Model

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This diagram shows six spectral colours, red, orange, yellow, green, blue and violet.

• Spectral colours are produced by a single wavelength of light.
• Some spectral colours can also be produced by mixing two wavelengths of light together.

Understanding the diagram:

• Above the line of coloured circles are the wavelengths of light that produce each spectral colour.
• Of the six colours (ROYGBV), red, green and blue can only be produced by a single wavelength of light.
• The other three spectral colours, orange, yellow and violet, can be produced either by a single wavelength of light or by additive mixing of pairs of primary colours.
• The bottom line of colours shows the proportions of red, green or blue used to produce orange, yellow and violet.
• The wavelength corresponding with each colour is shown in nanometres (nm). The wavelengths shown for each colour are for illustration only.
• In practice, the choice of wavelengths for primary colours usually depends on factors such as the colour model being used, the gamut of colours that a display device can produce and the context in which colours are to be viewed.
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## Description

#### Spectral Colour & the RGB Colour Model

###### TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
Yes! The spectral colours red, green and blue can be combined in pairs to produce orange, yellow and violet when using an additive colour model.
RGB is a colour model used to produce a full palette of colours by mixing red, green and blue light sources in different proportions.
Spectral colours are all the colours between red and violet that can be produced by a single wavelength of light. Sunlight is composed of spectral colours.
Yes! When spectral colours are arranged in a diagram, the order in which they appear corresponds with their wavelength. The easiest way to show this is a band of colours of increasing wavelength with red at one end and violet at the other.

• This diagram shows six spectral colours – red, orange, yellow, green, blue and violet.
• Spectral colours are produced by a single wavelength of light.
• Some spectral colours can also be produced by mixing two wavelengths of light together.
###### Understanding the diagram
• Above the line of coloured circles are the wavelengths of light that produce each spectral colour.
• Of the six colours (ROYGBV), red, green and blue can only be produced by a single wavelength of light.
• The other three spectral colours, orange, yellow and violet, can be produced either by a single wavelength of light or by additive mixing of pairs of primary colours.
• The bottom line of colours shows the proportions of red, green or blue used to produce orange, yellow and violet.
• The wavelength corresponding with each colour is shown in nanometres (nm). The wavelengths shown for each colour are for illustration only.
• In practice, the choice of wavelengths for primary colours usually depends on factors such as the colour model being used, the gamut of colours that a display device can produce and the context in which colours are to be viewed.
###### Remember that:
• The bands of colour we see in rainbows correspond with the spectral colours red, orange, yellow, green, blue and violet.
• Spectral colours are produced naturally when light is refracted and dispersed by a prism or by rain.
• A spectral colour is produced by a single wavelength of light.
• The visible spectrum contains a continuum of spectral colours between red and violet.
• The visible spectrum is a small part of the electromagnetic spectrum.
• In a continuous spectrum, separate colours may be indistinguishable to the human eye.
• The fact that we see distinct bands of colour in a rainbow is an artefact of human colour vision.
###### 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 spectral colour is usually presented in the form of a continuous linear spectrum organised by wavelength, so with red at one end and violet at the other.
• 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.

#### Some key terms

Primary colours are a set of colours from which others can be produced by mixing (pigments, dyes etc.) or overlapping (coloured lights).

• The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to the electromagnetic spectrum that results in the perception of colour.
• A set of primary colours is a set of pigmented media or coloured lights that can be combined in varying amounts to produce a wide range of colour.
• This process of combining colours to produce other colours is used in applications intended to cause a human observer to experience a particular range of colours when represented by electronic displays and colour printing.
• Additive and subtractive models have been developed that predict how wavelengths of visible light, pigments and media interact.
• RGB colour is a technology used to reproduce colour in ways that match human perception.
• The primary colours used in colour-spaces such as CIELAB, NCS, Adobe RGB (1998) and sRGB are the result of an extensive investigation of the relationship between visible light and human colour vision.

The trichromatic colour model is a theory of colour that establishes terms, rules and methods to enable human colour vision to be dealt with in both systematic and practical ways.

ROYGBV is an acronym for the sequence of hues (colours) commonly described as making up a rainbow: red, orange, yellow, green, blue, and violet.

• A rainbow spans the continuous range of spectral colours that make up the visible spectrum.
• The human eye is tuned to the visible spectrum and so to spectral colours between red and violet.
• ROYGBV are colours associated with a range of wavelengths rather than with unique values.
• The visible spectrum is the small band of wavelengths within the electromagnetic spectrum that corresponds with all the different colours we see in the world.
• The fact that we see the distinct bands of colour in a rainbow is an artefact of human colour vision.
• To be clear about the RGB colour model it is useful to remember first that:
• The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
• A spectral colour is a colour corresponding with a single wavelength of visible light, or with a narrow band of adjacent wavelengths.
• The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. However, because of the way the eye works, we can see many other colours which are produced by mixing colours from different areas of the spectrum. A particularly useful range of colours is produced by mixing red, green and blue light.
• RGB colour is an entirely different approach to producing and managing colour.
• RGB colour is an additive colour model in which red, green and blue light is combined in various proportions to reproduce a wide range of other colours. The name of the model comes from the initials of the three additive primary colours, red, green, and blue.
• Except for the three primary colours, RGB colours are not spectral colours because they are produced by combining colours from different areas of the visible spectrum.
• RGB colour provides the basis for a wide range of technologies used to reproduce digital colour.
• RGB colour provides the basis for reproducing colour in ways that are well aligned with human perception.
• When an observer has separate controls allowing them to adjust the intensity of overlapping red, green and blue coloured lights they are able to create a match for a very extensive range of colours.
• When looking at any modern display device such as a computer screen, mobile phone or projector we are looking at RGB colour.
• Magenta is an RGB colour for which there is no equivalent spectral colour.

The visible part of the electromagnetic spectrum is called the visible spectrum.

• The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
• As light travels through the air it is invisible to our eyes.
• Human beings don’t see wavelengths of light, but they do see the spectral colours that correspond with each wavelength and colours produced when different wavelengths are combined.
• The visible spectrum includes all the spectral colours between red and violet and each is produced by a single wavelength.
• The visible spectrum is often divided into named colours, though any division of this kind is somewhat arbitrary.
• Traditional colours referred to in English include red, orange, yellow, green, blue, and violet.

A colour model is the how-to part of colour theory. Together they establish terms and definitions, rules or conventions and a system of notation for encoding colours and their relationships with one another.

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.

A secondary colour is a colour made by mixing two primary colours in a given colour space. The colour space may be produced by an additive colour model that involves mixing different wavelengths of light or by a subtractive colour model that involves mixing pigments or dyes.

• Secondary colours produced by an additive colour model are quite different from the spectral colours seen in a rainbow.
• A spectral colour is produced by a single wavelength, or a narrow band of wavelengths, within the visible spectrum.
• A secondary colour produced by an additive colour model results from superimposing wavelengths of light from different areas of the visible spectrum.
• For the human eye, the best additive primary colours of light are red, green, and blue.
• A colour model, underpinned by a colour theory, provides a precise and replicable approach to understanding:
• How the human eye perceives light and interprets colour.
• Different types of colour such as the colours produced when mixing lights, pigments or inks.
• How to manage the diverse ways colour is processed by equipment such as cameras, digital screens and printers.
• Whether or not we recognize it, whenever we are working with colour, we are adopting a colour theory, a colour model and a colour space.