He refers 2:15 to a paper on tetrachromacy (people who have a 4th set of retinal cones, and are neurologically wired to use them.)
Proving someone has tetrachromacy requires asking a subject to mix red and green light until it looks identical to a reference yellow. Trichromats (normal people) all agree on one specific ratio. A tetrachromat, having a fourth cone type in between, perceives a subtle difference in those "identical" mixtures that trichromats can't, requiring a different mixing ratio entirely.
Although about 12% of women have this 4th cone type, only a very small proportion of them are able to use the extra come to distinguish more color types. ref
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