The color or colour between black and white. U.S. spelling has traditionally been gray, and British (or at least modern British) grey. The OED notes:
With regard to the question of usage, an inquiry by Dr. Murray in Nov. 1893 elicited a large number of replies, from which it appeared that in Great Britain the form grey is the more frequent in use, notwithstanding the authority of Johnson and later English lexicographers, who have all given the preference to gray . In answer to questions as to their practice, the printers of The Times stated that they always used the form gray ; Messrs. Spottiswoode and Messrs. Clowes always used grey ; other eminent printing firms had no fixed rule. Many correspondents said that they used the two forms with a difference of meaning or application: the distinction most generally recognized being that grey denotes a more delicate or a lighter tint than gray . Others considered the difference to be that gray is a ‘warmer’ colour, or that it has a mixture of red or brown …. In the twentieth century, grey has become the established spelling in the U.K., whilst gray is standard in the United States.
I would definitely agree with the last statement. The New York Times used gray more than 195,000 times between 1851 and 1980, and grey only 32,255. (The statistics are complicated by the fact that both spellings constitute a common last name.) However, my (American college) students almost unanimously choose grey. I hypothesize that this reflects the popularity of Grey Goose Vodka, designed for the American market and introduced in 1997, and of the TV show “Grey’s Anatomy,” which debuted in 2005. (The book to which the title refers is “Gray’s Anatomy.”)
Visiting the Tomb of the Unknown Soldier on a grey and misty morning, the President told thousands of mourners: “They fell so we might have the freedom, which too many of us take for granted, but at least on this day we know is still our greatest blessing.” (Time, May 29, 1995)/ The hills are shedding their summer gold for their fall grey. (San Jose Mercury News, October 13, 2011)