Science Questions

Are primary colours in pigment and light different?

Sun, 26th Feb 2012

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Neil S. Briscoe asked:

If you ask me what the primary colours are, I'd say Red, Green, Blue. They are often shown on TVs and Computer Screens.


Ask any artist, I've just had a conversation with one, and they will insist they are Red, Yellow, Blue - after all, you have to mix blue and yellow to get green.


A long time ago, a physics teacher tried to explain that light was additive whereas pigments were subtractive - but I need something concrete to explain to an artistic person why the difference.


So, why are the three primary colours different in light and pigment?





Dave -   This is a lovely problem and itís one of my bugbears, primary colours.  The primary colours of light are pretty much red, green and blue.  That's nothing to do with physics, itís all to do with biology.  In your eye, you've got three different types of cone cell, three different types of sensor:  Ones which absorb reddish light, ones which detect greenish light, and ones which detect bluish light.  Light is actually an incredible mixture of an infinite number of different colours, but your eyes approximate it to reddish, bluish and greenish.  So, if you mix red light and green light, you can actually confuse your eyes and make it look like itís yellow light.  And so, by mixing red, green and blue light, you can make any colour of the rainbow.  Itís actually slightly more complicated but [you can convince your eyes that you're seeing] pretty much any colour of the rainbow.  That's how TVs work, using red, green and blue.  

But if you're dealing with printing or paints, you're doing something different.  You're adding colours together.  You're taking white light which has got all the colours of the rainbow in it and you're taking colours away.  So, if you've got red, green and blue shining down on a red piece of paper, all that comes back is red light and you see red.  

The primary colours in that sense aren't made by adding colours together, like with light, but actually subtracting, or taking colours away.  So the primary colours with pigments are:

  • Taking red away; which actually emits a turquoise colour or cyan
  • Taking green away, which is basically purple, and
  • Taking blue away which is actually yellow.

So the primary colours of pigments are cyan, magenta, (purple) and yellow.  Red, yellow and blue are not any kind of primary colours at all and itís just primary school teachers trying to confuse you.

Chris -   Itís quite intriguing isn't it, because it is literally down to what is going on on a surface in order to make a colour that you see.  White light hits the surface, all the other wavelengths get absorbed apart from the one that you see coming back to you.  Whereas, if I shine light at you, I'm making some coloured light that your eyes interpret as colour I shone at you.

Dave -   Yes, itís to do with how you're getting those final colours which hit your eye.


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Colour printing is a little more complex as you do not want to use the three expensive colour inks to produce black  which would come more like a dark brown anyway so in addition to the yellow, magenta, and cyan seperations you produce a black one.
The three colur inks are not very pure so a complex system of additions and subtractions is used to compenste this being one of the last uses of anologue computers.  syhprum, Tue, 28th Feb 2012

The primary colours for light are red, green and blue, and it's easy to play with them on computer screens to combine them to make other colours. I wrote a little program to do this a couple of years ago which you can find here:

With paint it's easy to think the primary colours are red, yellow and blue because magenta does look nearly red and cyan looks nearly blue, and artists often use red instead of magenta when mixing their colours, as well as using a light shade of blue in combination with yellow to make green (which only works if that shade of blue approximates to cyan).

The two sets of primary colours are very clearly related to each other, as you can see if you use my program again and set two of the values to 255 and the third to 0 - this will allow you to create magenta, yellow and cyan because magenta is a combination of red and blue, yellow is a combination of red and green, and cyan is a combination of green and blue. With light, you add two dark colours to make a new lighter colour, whereas with paint you add two light colours to make a new darker one.

While on the subject of mixing colours to make new ones, you might be interested in metamers: a TV or computer screen will make a false yellow made out of red and green light, but you can also get pure yellow light which contains no red and no green light. Our eyes (and our cameras) can't tell the difference, but there are some creatures with as many as eight different colour sensor types in their eyes, and they would see our photographs as being highly defective. David Cooper, Tue, 28th Feb 2012

Primary colors of pigment are different from the red, yellow, blue primary colors of color theory. ak, Wed, 11th May 2016

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