heres what google says:
Gray and grey are both common spellings for the various neutral shades of color between black and white. Gray is more frequent in American English, and grey more common in Canada, the UK, and elsewhere. This pattern extends to specialized terms such as animal species (gray/grey whale) and scientific designations (gray/grey matter). Greyhound, however, is an exception; its grey shares a lineage with an Old Norse word for a female dog.
