What limits the color space in a Munsell system?


Why is the three dimensional representation of the Munsell color system not a cylinder? Why is it not even symmetrical when you move across different hues?

Is it not physically possible to create the colors outside of this range with the materials we have on earth? Are the colors outside not visible to the human eye, or are they basically indistinguishable from other colors already in the space?

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