Are you kidding? There have been hot debates about that! My theory (which no one ascribes to) is that Americans took in 'grey' reading Tolkien, and even if they totally rejected everything else about the reading experience of LOTR, all that elven grey came to mean something down, down in the hindbrain where fantasy works. Heck, I used it myself for years and years, until I got swatted for a British spelling in my text, by my very first copyeditor, and only then did I become conscious of it.

If I want to qualify my grays, I employ adjectives, but other writers continue to insist on using 'grey' and 'gray' in the same text, and expect readers to pick up the difference in hues.