I think Grey is a colour and Gray is a surname but i have been known to be wrong
(Warning! I have often wondered about and teetered between the two spellings. As I got into this, I found myself getting absorbed in it and have now written the following tedious comments.)
"Gray" is the more common, but not exclusive, spelling for the color in the US and for other US-English-speaking peoples. The opposite being true for other English-speaking peoples. Neither should be considered wrong, wherever. Either spelling is found in surnames regardless of turf, as in Zane Grey (American writer who was born "Gray", but the family soon changed the spelling of their name), or Andrew Gray (1st Lord Gray of Scotland).
Richard L.