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?
Now some of these results seem partly explicable: "Y" is yellow, maybe, because of the word "yellow" starts with "Y". That might also work for "R" red, "B" blue, and "G" green. For "A" I think of the big red apple with the "A is for apple" posters that ubiquitously decorate kindergarten classrooms. But "O" is not particularly associated with orange in this chart, nor "W" with white. And why are "X" and "Z" black? Because we're tired because it's near the end of the alphabet and our eyelids are starting to droop doesn't seem like a good answer. (Does it?)
You might wonder whether it's only synesthetes who have this consensus of associations, and how stable such associations are over time or between countries.
You're in luck, then, because here's another cool chart, from Australia in 2005!
The colored bars are synesthetic respondents and the hatched bars are non-synesthetic respondents. The patterns are similar between synesthetes and non-synesthetes, but maybe with the non-synesthetes tending toward stronger associations between the color and the initial letter of the color word. Furthermore, again "A" is red, "I" is white, and "X" and "Z" are black. US and Australian synesthetes seems to agree that "O" is white, but the Australian non-synesthetes like their "O" orange. For some reason, "D" is now brown (47%!).
There are some older US data from the underappreciated early introspective psychologist Mary Whiton Calkins in her classic 1893 paper on synesthesia. [Pop quiz: Who are the only three people to have been president of both the American Psychological Association and the American Philosophical Association? Answer: William James, John Dewey, and Mary Whiton Calkins.] She reports that synesthetes tend to associate "I" with black and "O" with white. "O" being white matches the synesthete reports from the US and Australia in 2015 and 2005, but Calkins's black "I" is different. Calkins reports this possible explanation for the whiteness of "O", from one of her participants, seeming to find it plausible: O "= cipher = blank = sheet of white paper".
Witthoft et al. 2015 found that almost a sixth of their participants born in the US in the late 1970s (but not those born before 1967) seem to have letter-color associations that match much better than chance with the colors of the letters of this then-popular magnet toy:image source]
Neat finding. Of course, the darned toy has "X" purple and "Z" orange, so it's all wrong!
Brang, Rouw, Ramachandran and Coulson 2011 find a weak tendency for similarly-shaped letters to associate to similar colors in US sample. Irish-based Barnett et al. 2008 and British-based Simner et al. 2015 find broadly similar patterns to the other recent English-language populations.
Spector and Maurer 2011 find that even pre-literate English-speaking Canadian toddlers associate "O" and "I" with white and "X" and "Z" with black, though they do not share older participants' associations of "A" with red, "B" with blue, "G" with green, and "Y" with yellow. They hypothesize that jagged shapes ("X" and "Z") might be more likely to have shaded portions in a natural environment than non-jagged shapes ("O" and "I"), and that other, later associations might be language based. However, color maps of Swiss research on German-language synesthetes (Beeli, Esslen, and Jaencke 2007) shows no such relationship (see the chart on p. 790) -- for example with more participants associating "X" with white or light gray than with black or dark gray (though Simner et al. have a German subset which do show black associations with "X" and "Z"). Beeli et al. find a weak tendency for higher frequency letters to be associated with higher saturation colors in a German-language sample. Rouw et al. 2014 found that Dutch and English-speaking non-synesthetic participants had similar associations for "A" (red), "B" (blue), "D" (brown), "E" (yellow), "I" (white), and "N" (brown). Hindi participants, with their different alphabet, had a rather different set of associations -- though the first letter of the Hindi alphabet was also associated with red. They speculate that the first letter in each alphabet gets a "signal" color.
Okay, so now you know!
Let me leave you then, with this highly unnatural thought: