But by 2010, [Jordan] had found a subject who perfectly acted the part of a tetrachromat. Jordan’s “acid test” involved coloured discs showing different mixtures of pigment, such as a green made of yellow and blue. The mixtures were too subtle for most people to notice: almost all people would see the same shade of olive green, but each combination should give out a subtly different spectrum of light that would be perceptible to someone with a fourth cone. Sure enough, Jordan’s subject was able to differentiate between the different mixtures each time. “When you ask them to discriminate between the two mixtures, a tetrachromat can do it very quickly. They don’t hesitate,” says Jordan.

Science has a method for cutting through speculation - in this case, it is a variant of the Ishihara test.

I'd use a 2-panel diagram. On each round, the panels are assigned random colors. The subject is asked "are the panels same, or different?" Some proportion of the time, they're identical. If they just mash "different" every time, you'll see that in the data and you'd reject those subjects -- or accumulate those false guesses into an uncertainty measure. If they hit 'same' on different colors... make a graph with edges between those colors. Some time after the graph is connected, try and find a minimum vertex cover. The order if that vertex cover should be a reasonable estimate of the number of colors a person can see.