I forgot to add that in addition to being non-spectral, they are the most saturated colors of their hue. All colors inside the horseshoe are non-spectral; they

I forgot to add that in addition to being non-spectral, they are the most saturated colors of their hue. All colors inside the horseshoe are non-spectral; they are mixtures of spectral colors. https://en.wikipedia.org/wiki/Line_of_purples https://en.wikipedia.org/wiki/Line_of_purples