History of Gray/Grey
The modern day spelling of gray (grey) actually comes from the Old English grǣg. Throughout the centuries there have been many variations in spellings, ei and ey and ai and ay.
There have also been attempts to distinguish grey and gray into different colors or hues, with the color gray being a simply black and white mixture but the color grey being slightly bluer. Perhaps this is why national surveys in both England and the United States show that many people believe these are actually two different hues, even though they aren’t.
Despite the complicated history, all you need to know is that they are used for different audiences.
Remember the Difference
A great trick to remember when to use which of these two words is that Gray is used in America and Grey is used in England.
Summary
So, how do you spell grey? Well, even though people get confused on the gray vs. grey debate, they can actually be used interchangeably. It is still important, however, to keep your audience in mind.
I clearly don't care about my audience. Sorry everyone. I know that in the past there has been discussion about who is a "must read" (especially prior to 4/16). My posts are more clearly a "forced to read" rather than "must read". My apologies.