Green Lion Art of Colour
Alchemy is at root an art of transformation, changes often accompanied by an alteration in colour. Alchemists formed an important scientific link in Europe between the "enlightened" scientific Greek / Roman cultures and the Renaissance.
When the economist John Maynard Keynes bought some of Newton's manuscripts in 1936 when Newton's papers were unfortunately dispersed, he drew attention to the non-mathematical, irrational side of Newton. Here was a famous scientist who had spent an equal part of his time, of not the major part, on a chronology of the scriptures, alchemy, occult medicine and biblical prophecies. For Keynes, Newton had been the last of the magicians.
The experimentum crucis, conducted in 1665 or 1666, of Isaac Newton lead him to the deduction that "light itself is a heterogeneous mixture of differently refrangible rays", following on from which went on to demonstrate that rays, separated by their passage through a prism, were "uncompounded colours" which could not be further split by a second prism. Passed through a lens, they merged back into a beam of white light. White sunlight is more than three primaries, and the idea of uniting the spectrum in a loop: a colour wheel, occurs in Newton's Opticks (1704), matching up red with violet by way of a colour not "of the prismatick Colours, but a purple inclining to red and violet". Newton gave no particular prominence to the hues regarded today as primary, and his wheel was a seven-spoked device composed with unequal slices.
...as Vincent Van Gogh (1853-1890) explained when quoting from Eugène Delacroix (1798-1863), in a letter to his brother Theo.