The optimal subtractive system primaries are cyan, magenta and yellow (with the secondaries being red, green and blue) though the artists’ colour wheel (which is like the painters’ wheel above) has red, blue and yellow as the primaries. Orange is not in sight – and yet surely if we are to make an argument for inclusion in the spectrum based on primaries (and/or secondaries) then it is the additive system that we should be using since the spectrum is emitted light. The additive secondaries are cyan, magenta and yellow. Additive mixing – – describes how light is mixed and the additive primaries are red, green and blue. The first thing I would have to say is that this argument seems to ignore the difference between additive and subtractive mixing.

This is wrong on so many levels it is hard to know where to start. It is argued that since six of the colours in the rainbow are primary or secondary colours in the colour wheel and indigo is not, then indigo has no right to be there. In this so-called painters’ wheel the primary colours are red, yellow and blue and the secondary colours are orange, green and violet. The group shows the following colour wheel: Interestingly, the facebook group referred to above would like to eject indigo from the spectrum on the basis that it is not a primary or secondary colour but rather a tertiary colour. Many modern commentators claim that only six distinct colours can be observed in the rainbow. It has been suggested that, at the time, Newton was trying make some anology with the musical scale and the octave (with its seven intervals) and hence was keen to identify seven colours in the rainbow or visible spectrum. It was Newton who suggested that the rainbow contains seven colours: red, orange, yellow, green, blue, indigo and violet. There is even a facebook group called “Get Indigo out of the rainbow”.

From time to time I come across web pages and groups of people who get irrate about indigo being in the rainbow.

Blue and indigo