There is no "gray" in the Wikipedia. It is "grey" since that has been used since AD 700, and "gray" only came into use by America since 1825.
http://en.wikipedia.org/wiki/Grey But if I had to say anything about either spelling which is correct it would be that "grey" is the color tending towards black (dark) and that "gray" is the color tending towards white (light).
Thus to me "grey" is the darker side of a color while "gray" is the lighter side of a color that defines all colors of that sort between black and white.
"S" is spelled "ess" because that is the way that you pronounce it, it is not "S" it is "ess".

Also with "grey" it is about how a person pronounces it, either "grey" like a short "a" (ah) or "gray" with a different pronunciation with a more longer "a" sound (aa).
Also depicts emotion as in "The sky is tending towards grey" as in getting darker, horror, heaviness, foreboding, eerie and perhaps, while "The sky is tending towards gray" depicts the sky as lightning up in the future and towards the lightness of the day and brighter.