Its unclear to me why the color space is 2-dimensional. Why wouldn't it be a 3-dimensional space, indexed by how much each of the 3-cones is activated ? Not clear to me from the article!

It is 3 dimensional. That commonly repeated CIE diagram is a 2d slice of the color volume. Since 1931 that diagram is obsolete, misleading, and fails at a lot of modern color science, and has been replaced many times, but is what many people go to. The most recent replacement (well, by CIE), is CIE 2015. Comment on it [1]

Modern color modeling is much richer then 3 parameters, because human vision is much more complex than simply color frequencies. CIE 1931 was low brightness, 2 degree field of vision, center of vision derived. As brightness increases, color perception shifts. Colors are NOT linear; sRGB and CIE 1931 chose such a small section of human vision that they approximate that section with a linear assumption. Modern CIECAM models are not linear, are not 3 parameter, because color is not linear (CIECAM02 is 6 parameter [2], there are several after that one). A century of experiments, wide color gamuts, HDR, have thrown out CIE 1931 as a good model. It’s only momentum now, and slowly higher end things are replacing it.

A good introduction is Color Appearance Models, by Mark Fairchild, also any of his technical papers give a starting point into the science.

[1] https://community.acescentral.com/t/cie-2015-cmfs-what-would...

[2] https://en.wikipedia.org/wiki/CIECAM02

It is, inasmuch as we have 3 types of cone, which is an inherent orthogonality. It is also not, inasmuch as each cone is a wavelength in the same spectrum.

Either way, you can project a volume onto a plane, which is great for communicating visual data on paper or screen.

The interesting question is "why that arc in particular"; my ignorance will shine through if I speculate.

I assume that the projection encodes something about our relative perception of each cone's band, hence the big green corner.

>indexed by how much each of the 3-cones is activated

This will actually differ from person to person. If you look at a pure yellow wavelength light next to a red/green light mixed such that they create the exact same perceived yellow to you, it will look different to another person.

Aside from that, not really sure what a 3d view with the dimensions being r,g,b would actually offer

There are three cones, but there is an additional constraint that we plot the colors at maximum summed luminosity. So for one cone you would just have a point; two would show a line from 0% cone A+100% cone B -> 100% cone A; three is a plane

I guess it is the 2-dimensional section such that it have constant total brightness. You can then multiply later by your desired brightness.