FWIW department - found online

It appears that the "gray" varient appeared later, as an American
English varient of "grey". Both have mostly the same many meanings:

"gray1 also grey    ( P ) Pronunciation Key (gr)
adj. gray·er, also grey·er gray·est, grey·est
Of or relating to an achromatic color of any lightness between the
extremes of black and white.

Dull or dark: a gray, rainy afternoon.
Lacking in cheer; gloomy: a gray mood.

Having gray hair; hoary.
Old or venerable.
Intermediate in character or position, as with regard to a subjective
matter: the gray area between their differing opinions on the film's
morality.

n.
An achromatic color of any lightness between the extremes of black and
white.
An object or animal of the color gray.
often Gray
A member of the Confederate Army in the Civil War.
The Confederate Army. "

Dictionary.com
[ http://dictionary.reference.com/search?q=gray ]

So yes, "grey" is a color, and was always so. Us Americans had to go
and be different with "gray."