Munsell first tried to arrange his colors in a sphere, but noted that “the desire to fit a chosen contour, such as the pyramid, cone, or cube, coupled with a lack of proper tests, has led to many distorted statements of colour relations”. Essentially, Munsell realized that his color solid had to have an irregular shape to fit his colors.

https://programmingdesignsystems.com/color/a-short-history-of-color-theory/index.html