Well I spell it gray. When I type them both on Microsoft word, grey gets a little red line underneath it (to show that it's misspelled) unless it's capitalized.
Grey. I hate it when ignorant people think I'm spelling it wrong when I know I'm not.
The reason it's underlined is because you can set Word to different versions of English. Yours would be (USA) English. If you set it to (Canada) English gray would be underlined in red.
Wait, what? So it depends on the language (English/Canadian opposed to American) whether it is gray or grey? Dang it! Took me forever to remember to spell it gray instead of grey. I feel americanized...
Grey is the original. I spell it that way... but that's just cause I'm in a commonwealth country. Those cheap Americans change what ever they get their hands on.