The Color Wheel
Newton's Opticks first described spectral hue relationships in terms of a circle, which located purples (which Newton described as "more bright and more fiery" than spectral hues) between spectral red and violet. Newton used this circle to explain the colors resulting from proportional mixtures of any of his seven spectral hues in additive color mixture. (Newton's circle of colors).
Within two decades his circle was adopted by artists as a "color wheel" to explain color mixing with paints, or subtractive color mixture. During the 19th and early 20th centuries the circle was displaced by the colorimetric triangle (each corner for one of three primary colors) instead, but today the circle is again the dominant geometrical metaphor.
Historically, painters and printers (and color theorists relying on paint or ink mixtures) based their color wheel on the RYB primary colors (red/yellow/blue). This wheel was divided into sixths by the addition of three "secondary" paint mixtures (orange, green, and purple) created by the mixture of two primary colors in equal proportions. A third category, "tertiary" colors, was originally conceived as any color containing all three primary colors (in effect, muted or grayed colors), but contemporary practice is to define them as the mixture of a primary color with a neighbor secondary color; these are located on the color wheel between the primary and secondary colors in the mixture.