A great many textbooks state that there are three colour primaries. This is normally followed by the statements:

1. All colours can be made by mixing together three primaries.
2. The primaries – which are often cited as being red, yellow and blue – are pure and cannot be created from mixture.

Not only do I profoundly disagree with these last two statements but I disagree with the statement that there are three colour primaries. Here’s why:

It is relatively easy to go into a lab or studio, start with three colours (any three; you pick’em) and find that you cannot make all colours. People who do this will often say, that theoretically you can make all colours from, say, red, yellow and blue but that practically you can’t simply because the primaries are not pure enough. The problem is, the more pure you make the primaries, the fewer colours you can make!! The fact is you cannot make all colours from three primaries no matter how carefully you choose the primaries. You cannot do it practically and you cannot do it theoretically.

We can trace the idea that primaries are ‘pure’ back to ancient Greece. In those times and for centuries afterwards it was even frowned upon to mix colours at all because of the loss of purity.

It turns out that if you want to make a large range of colours using three inks or paints, the primaries you should choose are cyan, magenta and yellow. Don’t just take my word for it. Go and ask HP, Canon or Xerox. These companies have made printers for decades and make a living out of selling devices that allow consumers to make a wide range of colours with just three primary inks. They all use cyan, magenta and yellow as their primaries.

But how can magenta be a primary you might ask? It’s far from pure. That is because the notion of primaries being pure is an outdated idea (outdated for several centuries I might add) and should not be taught in Schools. Cyan, magenta and yellow make good primaries for an ink system precisely because they are not visually pure – they each absorb in a narrow part of the visible spectrum and therefore emit light quite broadly. Blue would make a poor primary in an ink or paint system because it absorbs at too many wavelengths. Mixing together blue and red inks make a very dirty brownish black colour. So the gamut (the technical term for a range of colours produced by some primaries) of colours we can make from red, yellow and blue inks or paints is quite small.

So, we can’t make all colours from three primaries, the best primaries are not those that are pure, and primaries can be made by mixing other colours. It is easy to show that a blue can be made by mixing together cyan and magenta inks and this is shown rather nicely by the artist Scott Naismith in this very nice youtube video.

We tend to use three primaries in many systems because you can make a great many more colours with two primaries than with one and you can make a great many more colours with three primaries than you can with two. But you can’t make all colours with three. You can make more colours (a larger gamut) with four or five primaries – though you still can’t make them all – but we reach the point of diminishing returns. Is it worth the extra expense of having four or five primaries if three do a pretty good job? Usually not. However, sometimes we do think it is worth using more than three primaries. For a start, most printers use CMYK (cyan, magenta, yellow and black) so that is four primaries. Then we have hexachrome printing systems with six primaries. The Quattron TV is manufactured by Sharp and has four primaries (red, green, blue and yellow) whereas most TVs only have three (red, green and blue).

The truth is there is no perfect set of primaries and there is no fixed number. A set of primaries is simply a set of colours in a colour system that can make a useful range of colours (gamut). Very often three hits the commercial soft spot but that’s just about engineering and economics.

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