We normally think of there being three colour primaries. There is no one set of primaries of course. If we think of additive colour mixing the primaries are red, green and blue. But which red, green and blue? There are almost as many sets of RGB primaries as I have had hot dinners. And I like hot dinners.

In subtractive mixing we sometimes read that red, yellow and blue are primaries; though we know now that RYB results in a rather small colour gamut and is not a great choice of primaries. Techies may know that the optimum subtractive primaries are cyan, magenta and yellow and, indeed, CMY are the primaries used in most desktop printers (though, again, there is no one set that everyone uses).

However, there is a sense in which there are four colour primaries: these being, red, green, yellow and blue. These are the so-called psychological primaries. Whereas the other sets of primaries I mentioned relate to technology – the technology of colour mixing – the psychological primaries relate to our phemenological experience of colour more directly. Red, green, yellow and blue are opponent colours – red is opposite green and blue is opposite yellow as first formally descibed by the physiologist Ewald Hering in the late 1800s. We can think of a colour as being a mixture of red and yellow (i.e. orange) or red and blue (i.e. purple) but we cannot conceive of a colour that is a mixture of red and green. We don’t see red and green at the same time in the same colour. Furthermore, these four colours are the unitary or unique hues. Though one can see reds that are bluish and reds that are yellowish, there is red that if you saw it you would say it is a pure red, neither bluish nor yellowish. Surprisingly, perhaps, there is remarkable agreement between different people as to the exact unique colours. For example, if people are shown a spectrum of different wavelengths and asked to pick the green that is pure (without bias) they will also select almost the same wavelength. Orange is not unique in the same way – though some oranges are reddish and some are more yellowish, every orange looks like a mixture of red and yellow. Orange cannot therefore said to be a unqiue hue.

There is a particular power and salience about the four colours red, green, yellow and blue and therefore it is no surprise to find these colours used frequently in advertsing and design. Most spectacularly, they are all used at the same time in two of the most famous logos in the world.

 Curiously, Ruth Kedar, the graphic designer who developed the google logo was quoted in Wired magazine in 2008 as saying, “There were a lot of different color iterations. We ended up with the primary colors, but instead of having the pattern go in order, we put a secondary color on the L, which brought back the idea that Google doesn’t follow the rules.” It seems more likely, however, that the Microsoft logo is based on a clearer understanding of the relationship between the four psychological primaries. Though I have yet to find reliable information about the Microsoft logo so please add some details if anyone knows by making a comment.

 There currently seems to be a lawsuit between Microsoft and Google regarding the use of colour in these logos. For further information goto http://colormatters.blogspot.com/2009/07/color-infringement-microsoft-vs-google.html.