There are many reasons why I don’t like colour wheels of the type shown below:

The first reason is because it perpetuates the myth that the subtractive primaries are red, yellow and blue whereas the fact is that red, yellow and blue produces a rather small gamut of colours. It is certainly not the best choice of subtractive primaries though it is taught as dogma in many art and design schools and throughout children’s education. The problem is that whenever two colours are mixed together there is saturation loss; that is, the resultant mixture ends up being more desaturated than the two components were. The saturation loss is greatest when mixing colours on the opposite side of the colour circle where the resultant mixture can be almost grey. However, for certain choices of primaries, the saturation loss is greater than for others. If red, yellow and blue are used as the primaries then of course it is possible to generate any other hue. However, there is significant saturation loss and the above colour wheel gives completely the wrong impression. It suggests that mixing blue and yellow together, for example, results in a really bright vivid green.

The reality of pigment mixing is much more like the triangular colour wheel shown below:

In the above diagram it can be seen that mixing together yellow and blue results in a really muddy dark green. The purple resulting from mixing blue and red is almost black!! Now it is possible to mix together a blue and a yellow to get a better green. For example, mixing a greenish blue with a yellow will give a much more vivid green. Mixing a bluish red with a greenish blue will result in a lovely purple. We have a name for a greenish blue and a blueish red – we call them cyan and magenta. A much better colour gamut is obtained if we start with the primaries, cyan, magenta and yellow.

Footnote: Some people may look at the triangular colour wheel and think that the reason the colours are dull is that the red, yellow, and blue primaries used are not ‘pure’ enough. Nothing could be farther from the truth. If it was possible to make really vivid and bright red and blue pigments then the resultant colour gamut would be even smaller. Fundamentally, red, yellow and blue just don’t make good subtractive primaries.