Red-Green Color Blindness ...wait, wtf?
I recently reread an old Ellery Queen book (The Greek Coffin Mystery). One big clue is a blind man wearing a red tie that should have been green.See, he's a picky dresser, and he has a schedule written out for what he wears every day of the week. By the schedule, the tie should have been green, but he was seen wearing a red one. And he called his tailor and ordered six new ties - "just like this one," he told someone when he said he was planning to order them - and they sent red ones. Ergo, he knew the tie he was wearing was red.
Ellery spins a whole theory presuming the blind man can really see, which is just his first wrong guess. It turns out that the blind man's cousin, who acts as his valet, has red-green color-blindness... meaning, according to Ellery, that "he sees red as green" and "when Khalkis wanted a red tie he had to ask for a green one".
Leave aside whether that's how RG color-blindness actually works - that, instead of being unable to tell them apart, or even see them (here's nice explanation of what goes on with perception), Demmy actually saw two totally distinct shades. This would mean that when Demmy looked at grass he saw the wavelengths other people see when they look at a firetruck. That's not even close to how it works.
But as I said, leave that aside. This also means there's some innate connection between the word "green" and the color seen when you look at grass, so that when as a child Demmy was told that grass is green, he rejected that label and insisted that grass was red. That's ridiculous.