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Eyes, the macular and seeing yellow.
I was wondering if the reason Grant found that yellows came in brighter was to do with the macular. The macular is part of the retina but what's interesting in our case is that it's often found to contain high amounts of lutein, leading supplement sellers to conclude it must be needed there and we should of course supplement :/
Lutein is a carotenoid and a yellow pigment. This could mean it effectively adds a yellow filter over the retina. The human optic system is very good at colour correction to cope with a broad spectrum of lighting conditions, so you don't really notice this yellow accumulation in your field of vision except that yellow objects seem less yellow, because relatively speaking it is compared to the overall yellow filter.
So I speculate, that Grant's low retinoid diet has removed the supply of this reactive pigment to the Macular and turned down the yellow filter allowing him to see yellow relative to a much lower base line.
I feel quite smug about coming up with this, but surely someone has already done so. Anyone? Carry on.