Spectral Colour and the RGB Colour Model

Look carefully at the diagram at the top of the page. Now check out the following questions (and answers)!

1. What are spectral colours?
2. What wavelengths correspond with the extreme limits of the visible spectrum?
3. Does each wavelength of visible light correspond with a unique spectral colour?
4. Are rainbow colours spectral colours?
5. Are colours produced by combining RGB primary colours spectral colours?

Introducing the diagram! Read back and forward between the image at the top of the page and the explanation below!

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.