“In a mixture of primary colours, the quantity and quality of each being given, to know the colour of the compound.”
from wiki – Throughout Opticks, Newton compared colours in the spectrum to a run of musical notes. To this purpose, he used a Dorian mode, similar to a white-note scale on the piano, starting at D. He divided his colour wheel in musical proportions round the circumference, in the arcs from DE to CD. Each segment was given a spectral colour, starting from red at DE, through orange, yellow, green, blew [sic], indigo, to violet in CD. (The colours are commonly known as ROY G BIV.)
The middle of the colours—their ‘centres of gravity’—are shown by p, q, r, s, t, u, and x. The centre of the circle, at O, was presumed to be white. Newton went on to describe how a non-spectral colour, such as z, could be described by its distance from O and the corresponding spectral colour, Y.