Artist 
Writer Newton color system
The Newton color wheel was developed in 1666 and the colors represent the seven notes of the diatomic scale in music and the seven planets of classical tradition. The colors are red, orange, yellow, green, blue, indigo, and violet. This corresponds to the colors in the spectrum, which start with the shortest wavelength, red, and finish with the longest wavelength, violet. Red-violet or purple does not exist in the physical spectrum and is purely a subjective color. Newton came up with the idea of a circle by noting a visual resemblance between red and violet, allowing him to bring the two ends of the spectrum together to make a circle.
See the Color Building Block for more information.