ProPhoto RGB colorspace has room for some non-physical colors, but it still only has three axes for primaries. It just has a range broad enough to include most

ProPhoto RGB colorspace has room for some non-physical colors, but it still only has three axes for primaries. It just has a range broad enough to include most visible colors as well as some that don't correspond to a sensory input you can get with your three color receptors. With paints and pigments, it's pretty common to have more than three primary colors mixed in. But with more non-orthogonal axes, there's more overlap where you can achieve the same color multiple ways by combining the colors in different proportions. E.g. purple could be a single purple wavelength or a combination of blue and red.