This might be a good start: https://en.wikipedia.org/wiki/MacAdam_ellipse

Describes it exactly. Thank you!

Not sure, but maybe you could test by showing people two similar colors and asking them to decide if the are identical.

With enough samples (and enough individual participants) you establish how many "statistically-significant distinguishable differences" exist along a given path through the color-space.

Then combine those and get a sort of topographic grid...

Yeah, something like that was my best guess—either that or letting subjects adjust a second color until it matched the first. Turns out that's exactly what they did: "MacAdam set up an experiment in which a trained observer viewed two different colors, at a fixed luminance of about 48 cd/m2. One of the colors (the "test" color) was fixed, but the other was adjustable by the observer, and the observer was asked to adjust that color until it matched the test color." - from https://en.wikipedia.org/wiki/MacAdam_ellipse

That would actually be an interesting research topic because the results might be influenced by culture/language:

https://en.wikipedia.org/wiki/Blue%E2%80%93green_distinction...

It would be easier to measure the photoreceptor response of the eye and mathematically extrapolate.