What is "color solid"

A color solid is the three-dimensional representation of a color model, an analog of the two-dimensional color wheel. The added spatial dimension allows a color solid to depict an added dimension of color variation. Whereas a two-dimensional color wheel typically depicts the variables of hue (red, green, blue, etc.) and brightness (gradations of light and dark), a color solid adds the variable of saturation, allowing the sphere to depict all conceivable colors in an organized three-dimensional structure.