Hmm. I wonder how subtle tetrachromatism can be, and how unsubtle that test is. I have the other markers - colorblind people in my close family - and I've always noticed that I can see subtle gradations in color that other people can't. (I used to work in a print shop, so it came up. A lot.) That article talks about difference in the wavelength sensitivities of the fourth cone as the defining factor. I can test this!
Eeeeenteresting.