Grey is the British standard. Gray is the American version.
From Wikipedia....
grey gray Grey became the established British spelling in the 20th century, pace Dr. Johnson and others,[118] and it is but a minor variant in American English, according to dictionaries. Canadians tend to prefer grey. The non-cognate greyhound was never grayhound. Both Grey and Gray are found in proper names everywhere in the English-speaking world. Americans tend to differentiate spelling when comparing the color with the adjective.
We always use grey when discussing our calls. Calls came from Europe so we use that spelling. When I went through elementary school it was "grey."