I use “gray” unless I am submitting a paper to a journal that requires me to conform to British conventions.
@Lightlyseared “It comes from the old English græg… usually when the US simplified æ they went with e but in this case they favoured a.”
And if we go further back, the Indo-European word was “grēwaz.” At first blush, this would seem to favor the “e.” But interestingly, the Proto-Germanic word was grawja (which favors the “a”). In fact, British English was one of the only languages that ended up with the “e” over the “a.” And if you look at cognates in other languages, almost all of them went with the “a” (Dutch: grauw; German: grau; Old Norse:grár). Even Old Saxon favored the “a” and went with grāo. The only linguistic ancestor of English that favored the “e” is Old Frisian, which had the word grē. So in this case, favoring the “e” makes British English the outlier (not that this means it is any less correct—just different).
@kritiper “In my old The New Century Dictionary, from 1944, it lists “grey…See gray, etc.” and under “gray, grey” (note which is listed first)”
That’s because the New Century Dictionary favors alphabetical order. If there are two acceptable versions of a word, it lists both under whichever comes first alphabetically. Contrast this with other dictionaries, such as the New Oxford American Dictionary, which favors British spellings (so “gray” directs you to “grey”) or the American Heritage Dictionary, which favors US spellings (so “grey” directs you to “gray”). It’s all a matter of which organizing principles a particular compiler chooses to adopt.