Tone: The next circle of colors represent the tone of each hue. A tone is the hue mixed with true gray. The hue mixed with any amount of gray is considered a tone of the hue. On the color wheel only one tone is shown. This is the hue mixed with gray.
The tetrad relationship uses four colors that are chosen from every third color on the color wheel. Like the triad relationship, the name tetrad comes from the shape made between the colors. Orange/yellow-green/blue/red-purple is just one example.
Color Wheel (Moses Harris, Moses Harris's chart was the first full-color circle. The 18 colors of his wheel were derived from what he then called the three 'primitive' colors: red, yellow and blue. At the center of the wheel, Harris showed that black