You are probably familiar with the horseshoe-shaped chromaticity diagram [0] of human-visible colors. A light source with three color primaries spans a triangle in that coordinate system. To cover the whole horseshoe, at least one of the vertices would need to be way outside the horseshoe. With four color primaries, you get a quadrilateral that makes it easier to cover a larger portion of the horseshoe.

The reason the visible colors form a horseshoe rather than a triangle is due to how the cones’ sensitivity ranges overlap [1]. They cannot be excited independently by the primaries of a display.

[0] https://upload.wikimedia.org/wikipedia/commons/1/1e/CIE1931x...

[1] https://upload.wikimedia.org/wikipedia/commons/thumb/0/04/Co...

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summa_tech 3 hours ago | parent | next [-]

Excellent explanation.

I'd like to add that no light source can lie outside the horseshoe of the CIE xyz diagram: pure wavelengths are points on the curved line, everything that mixes them moves towards the inside of the space. So you're stuck with triangles that fit within it.

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layer8 3 hours ago | parent | next [-]

I was wondering about that, since some color spaces have their primaries outside the horseshoe. Thanks for clarifying.

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extesy 2 hours ago | parent | prev [-]

What does it mean then for ProPhoto RGB triangle to be outside of the horseshoe on that diagram?

	▲	AprilArcus 2 hours ago \| parent [-]
		It uses nonphysical "imaginary" primaries that have meaning within the coordinate system but not within the human perceptual system.

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gallerdude 2 hours ago | parent | prev | next [-]

This was a really fun visualization, so I vibecoded it.

https://www.jackgaller.com/colorspace

	▲	pohl an hour ago \| parent [-]
		Nice. If I add a primary and then clicking optimize—without first moving the primary—it seem as though there are still only 3 primaries, because the new one doesn't move.

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bbqbbqbbq 12 minutes ago | parent | prev [-]

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