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.