why red means red in almost every language

The results revealed two remarkable patterns, which Kay and Berlin laid out in their 1969 monograph, Basic Color Terms. First, almost all of the languages they examined appeared to have color words that drew from the same 11 basic categories: white, black, red, green, yellow, blue, brown, purple, pink, orange, and gray. Second, cultures seemed to build up their color vocabularies in a predictable way. Languages with only two color categories chunked the spectrum into blacks and whites. Languages with three categories also had a word for red. Green or yellow came next. Then blue. Then brown. And so on.
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Kay and Berlin took these commonalities as evidence that our conception of colors is rooted, not in language, but in our shared human biology.

Here.