When newton discovered the components of the spectrum he announced there were 7 colours. The number 7 was considered to be magical at the time and so he ‘saw’ a 7th colour to fit the number. When colour theorists started making colour wheels the indigo didn’t make sense, but with the number reduced to 6 it fell into place. The spectrum is a product of the way we see. We have 3 colour receptors in our eyes and the 3 primaries are their equivalent in paint. The brain then recognises the 3 intermediary hues as distinct basic colours. We see these as separate colours in the spectrum, and all colour mixing is based in the interaction between these 6 colours and the tones and shades of them.