Gray vs. grey
Gray and grey are different spellings of the same word, and both are used throughout the English-speaking world. But gray is more common in American English, while grey is more common in all the other main varieties of English. In the U.K., for instance, grey appears about twenty times for every instance of gray. In the U.S. the ratio is reversed.
Both spellings, which have origins in the Old English grǽg, have existed hundreds of years. Grey gained ascendancy in all varieties of English in the early 18th century, but its dominance as the preferred form was checked when American writers adopted gray about a century later. . . .[T}his change in American English came around 1825. Since then, both forms have remained fairly common throughout the English-speaking world, but the favoring of gray in the U.S. and grey everywhere else has remained consistent.
Some people make their own distinctions between gray and grey. . . There is nothing wrong with these preferences, but they are not borne out in broader usage. For most people, gray and grey are simply different spellings of the same word.