“A” Is Red, “I” Is White, “X” Is Black – Um, Why?
Check out this graph of the color associations for different letters for people with grapheme-color synesthesia. This is from a sample of 6588 synesthetes in the US, reported in Witthoft, Winawer, and Eagleman 2015. Presumably, they're not talking to each other. But there's a pretty good agreement that "A" is red, "X" is black, and "Y" is yellow. But you knew that already, right?
(Image used under Creative Commons license via this page.)
Prevalence of Learned Grapheme-Color Pairings in a Large Online Sample of Synesthetes
In this paper we estimate the minimum prevalence of grapheme-color synesthetes with letter-color matches learned from an external stimulus, by analyzing a large sample of English-speaking grapheme-color synesthetes. We find that at least 6% (400/6588 participants) of the total sample learned many of their matches from a widely available colored letter toy. Among those born in the decade after the toy began to be manufactured, the proportion of synesthetes with learned letter-color pairings approaches 15% for some 5-year periods. Among those born 5 years or more before it was manufactured, none have colors learned from the toy. Analysis of the letter-color matching data suggests the only difference between synesthetes with matches to the toy and those without is exposure to the stimulus. These data indicate learning of letter-color pairings from external contingencies can occur in a substantial fraction of synesthetes, and are consistent with the hypothesis that grapheme-color synesthesia is a kind of conditioned mental imagery.