The nature of white colour/light remained unknown until the Isaac Newton’s Optics was published. The prominent English physicist proved that the white light contains all other colours, and described his experiments with the prism that allowed seeing the spectrum. However, there is no sharp separation of colours in the spectrum (neither in the rainbow), and thus Newton tried to bring the number of colours to seven by analogy to seven notes of diatonic scale.

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That is what he writes about this in his "Lectures" (1669):

“Moreover, if all the other kinds of rays were considered simultaneously, that assertion, though no longer absolutely true, would still so closely approach the truth that it could be taken as true with respect to sense and a mathematical calculation. Consequently, since a geometric calculation of the refraction... can be undertaken rather difficultly, I will not be afraid to do it in a way which, however mechanical, is more suited to practice, being confident that no fault aught to be attributed to me if, when I perform computation in physical matters, I omit the minutiae, that entail burdensome and fruitless work.”

Review of order of colours in rainbow (spectrum) demonstrates that the dark-blue (indigo) was added to bring the quantity of colours to required one. After all, blue and dark-blue are the same colours per se, otherwise the rainbow should also contain pink, light-orange, crimson (i.e. any colour distinguished by the human eye). In addition, when spectrum is divided into 7 colours, the correlation and properties of these colours (which could help the artists) remain unclear.

In 1810 German artist Philipp Otto Runge (1777-1810) published his theory about colours, in which he indicated 3 basic colours - yellow, blue and red. By mixing these colours one can obtain orange, green and violet.
The modern chromatics employed by the artists is based on this three-dimensional colour model. Colours are arranged in the form of circle subject to colour contrast and colour combinations.

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Why a note can not correspond to a colour?

What is a consonant chord and consonance intervals?

Why the intervals are to be given the numerical values of semitones?

Why the interval of 3 semitones is compared with the blue colour and not with yellow one, for instance?
How the multitude of colours we see is expressed in music?

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