Are primary colours in pigment and light different?

26 February 2012

Question

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?

Regards
Neil

Answer

Dave - This is a lovely problem and it's 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, it's 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 it's yellow light. And so, by mixing red, green and blue light, you can make any colour of the rainbow. It's 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 cyanTaking green away, which is basically purple, andTaking 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 it's just primary school teachers trying to confuse you.

Chris - It's 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, it's to do with how you're getting those final colours which hit your eye.

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