The Munsell system, which attempts to represent pigment colors, has become a standard method of specifying paint pigments. The system is primarily a visual one, which  reflects the psychological aspects of color rather than the physical.

Munsell's color tree shows 20 hues, each represented by one
branch of the tree. The brightness or value of each square is
indicated by its vertical position (light at the top and dark
at the bottom) and the saturation of each hue and thus its
chroma is indicated  by how close or far
the square is from the trunk of the tree.

Gradations in saturation are presented as mixtures
of hues with their complementary colors,
based on the opposite hues in a
10 or 20-point color circle.

A 10-point circle of the colors of the spectrum with three non-spectral complementary colors.

Munsell's 20-point circle in which each color is opposite to its complimentary color.