Department of Physics and Astronomy, University of Glasgow
Department of Physics and Astronomy
Physics & Astronomy » Exhibits » The Colour of light
additive colour mixing

Additive colour mixing: where 2 or 3 of the coloured light circles overlap the colours "add up".

Colour mixing

The colours we see can be generated in two different ways.

Additive mixing
Starting with black (i.e. no light) and adding light in the additive primary colours red, green and blue in different ratios, any colour can be created. For example, green plus red (but no blue) gives yellow. Your computer screen on which you are reading this uses precisely this method to display different colours.
Subtractive mixing
Starting with white light and subtracting different amounts of the subtractive primary colours cyan, magenta and yellow can also create any colour. The pigments in colour printouts and paintings don't reflect some colours: they subtract those colours from the reflected light, so colours on the Mona Lisa's intriguing expression result from subtractive mixing.

Try it out, either with the interactive applet below or in our department's entrance hall!

Your browser does not support Java, so nothing is displayed. colour mixing in action

Colour mixing in real life: the interactive exhibit in our department's entrance hall.

the car door problem

A very practical problem

Imagine that your local workshop re-sprays your car door in glorious vivid green. It matches the colour of the rest of the car perfectly, at least in the dealer's garage, where the lights are more yellow than outside. However, as you drive outside the door shows its true colour: the workshop used cyan paint, which can look green in yellow light.

This illustrates that lighting conditions also play a role which is particularly important when trying to match two colours. Try this out with the interactive Javascript program below, which simulates how we would see coloured stripes in different lighting conditions. Specifically, try out how cyan looks green in "red and green" light.

              

Here are the results from a little experiment we did. We took photos of University of Glasgow folders, which come in a range of colours (and make an ideal gift for any occasion):
University of Glasgow folders, illuminated with white light

illuminated with white, ...

University of Glasgow folders, illuminated with blue light

blue, ...

University of Glasgow folders, illuminated with red and blue light

red and blue ...

University of Glasgow folders, illuminated with green and blue light

and green and blue light.

 

Explore further

  • Sir Isaac Newton's work on colour and other areas of physics, from BBCi
  • additive and subtractive colour mixing basics at wikipedia.org
  • colour vision and some fun things to try at colormatters.com
  • Javascript shape-drawing code at walterzorn.com

Credits

  • exhibit built by Alan Cumming, Craig Paterson and Graham Gibson
  • web page written mostly by Alan V. Cumming
  • colour-mixing applet written by Johannes Courtial
  • special thanks to Jim Jamieson of SSERC for supplying the basic apparatus
University of Glasgow Department of Physics and Astronomy