Wrong primary colours

[chakka35]

The primary colours are red, blue, and green, the three (approximate) colours that the human retina is most sensitive to. Light entering the eye will be recognised as a particular colour according to the proportions of R, G and B it contains. The only everyday use of this principle � mixing light � is in colour television, where the three phosphors in each pixel of the CRT screen glow with these three colours � glowing equally strongly for white because R+G+B = WHITE.
If you subtract R, G and B in turn from WHITE you get their complementary colours which are, respectively, Cyan, Magenta and Yellow. These are the "artist's primaries", the three colours needed in inks, paints and dyes (and the three you would have to take onto your desert island if you were not allowed more) to produce any colour under the sun. (You can see dots or squares of them in the centre-fold of newspapers nowadays.)
End of lecture; now for the question. As a child I was wrongly taught (as I suspect everyone else was) that the primary colours are red, blue and yellow. Astonishingly, people are still being taught this! For example see the BBC site www.bbc.co.uk/homes/design/colour_wheel.shtml. Would some educationalist please explain how this misinformation came about and why it is still being promulgated.

[kempie]

As a child did you have access to magenta and cyan crayons and paints?

When you are very young and are using paint or crayons, if you need to create a colour that you do not have, then of course you should mix the right proportions of the subtractive primary colours - cyan, magenta and yellow. However, it is very rare that you will find cyan or magenta paint or crayons amongst the selection given to a young child, so your teacher helpfully advises you to use red as an approximation to magenta, and blue as an approximation to cyan. And that's it, from that moment on a young child believes that they can mix red, yellow, and blue to make any colour, and that they therefore must be primary colours. As this concept is instilled in them from such an early age, it is often very difficult to convince them otherwise.

[Bert]

Chakka and Kempie are both confusing the mixing of light with the mixing ot pigments. When you mix the three primary colours of light (red, green and blue) you get white. When you mix the three primary colours of paint (red, yellow and blue) you get (allegedly) black - but it all depends on what sort of red, yellow and blue you start off with. You will usually get a filthy brown sort of colour. Whereas you cannot get a green light by mixing other colours of light (that's why it's a primary colour), you can get a green paint by mixing yellow and blue paint.

[Llamatron]

Surely Cyan and Magenta are more specific shades of the colours more generally known as Blue and Red?

So nobody is wrong.

[ugly_bob]

not really Llamatron.

Cyan and magenta (along with yellow) are primary colours for paint/printing/dyes. But, like kempie says, you don't really get these as crayon colours, so red and blue are used as approximations.
Printers used Cyan,Magenta,Yellow and Black (CMYK) to produce the colours they can (and more recently LC and LM to more accurately represent skin tones etc). A bright red is usually considered to be 0%C, 100%M, 100%Y, 0%K and a fairly standard dark blue is 100%C, 100%M, 0%Y, 0%K

[chakka35]

Yes, kempie, I had thought of that explanation but I am not convinced. I do realise that one sometimes has to approximate for a small child when the truth is too complex. But that is not the case here. A child would have no more diffculty in learning �cyan� and �magenta� than in learning all the other colours. And if manufacturers included a cake of cyan and magenta in the paint-box (to add to the yellow which would already be there) then parents and teachers could tell the child the truth from the start. Also, when we do approximate we correct the situation when the child gets old enough to understand. We do not do that in this case, hence the mililons of adults (like Bert whom I�ll answer below) who still talk of red, blue and yellow as being the primary colours. I still don�t understand it. It seems to be a case of adults passing on their misapprehensions to their children � the colour-blind leading the colour-blind, perhaps?

[chazza]

The primary colours of light are red green and blue

The primary colours of paint etc are red blue and yellow, according to what we were taught in school

[chakka35]

Sorry, Bert, but you�re wrong. I�d have thought that my original post made it clear that I know the difference between mixing light (additive mixing) and mixing pigments (subtractive mixing), and for the latter you definitely need the three complementary colours (cyan, magenta and yellow) if you are to produce every colour recognisable to the human eye. You are bound to get a muddy colour if you mix red, blue and yellow because they�re the wrong colours to produce black. If you mix cyan (which absorbs the red from white light) with magenta (which absorbs the green) and yellow (which absorbs the blue) you will have nothing left � pure black! You certainly get a shade of green if you mix blue and yellow, but for pure green you need cyan (which absorbs the red) and yellow (which absorbs the blue) leaving the pure primary green.
Again I suggest you look at the dots in the centre-fold of your newspaper; you will find them labelled C,M and Y (plus K for black). These are also the colours of the inks in the colour cartridge of your computer printer. Take pencil and paper and work it out for yourself. Better still, get a good text-book on the subject.

[heathfield]

Anyone remember those flat spinning tops that had three geared discs on them? The discs were each divided into thirds, coloured 'primary' red, yellow, and blue.
As you spun the top, the discs rotated at different speeds as they were whirled round. Changing bands of every colour spread across the surface. Beautiful to see, and something I've not ever forgotten.

[kempie]

In lieu of a good text-book...

http://en.wikipedia.org/wiki/Primary_color

[rojash]

All these arguments about the primary pigments for subtractive mixing (i.e. for painting, printing etc) are easily resolved. What colour inks do you put in your printer? Cyan, Magenta and Yellow. What are the print processing colours known as? CMYK = Cyan, Magenta, Yellow and blacK. How many kids have heard of Cyan and Magenta? They learn the "colours of the rainbow": red, orange, yellow, green,blue, indigo, violet. So - at school they are taught red, green and yellow.

[rojash]

Oops! menat Red Blue and Yellow, of course!

[chakka35]

Yes, rojash, that's what I'm complaining about.

[rojash]

Yes, chakka35 I was trying to address too many issues in a single answer, I think. The first part of my answer (CMYK etc) was to those who seemed to have difficulty understanding the difference between Cyan and Blue, and Magenta and Red.

The second part (colours of the rainbow) was to address your remark "A child would have no more diffculty in learning �cyan� and �magenta� than in learning all the other colours". Think about primary school and what happens there: they sing songs about rainbows, and that's where they usually take their first steps in painting and learning about (subtractive) mixing. The issue is probably not addressed again (if at all) until they start their science curicullum.

As far as I recall, it didn't even crop up explicitly in physics when I was at school. I only learned the difference because I happened to choose television and the cathode ray tube as a homework project when I was about eleven or twelve.