I sometimes find myself using certain words, in written communications, where I know that the British spell those words differently than Americans, but I can't always remember which spelling is supposed to be the "American" spelling.
That isn't an issue with all words, of course. For example, I've always known that "colour" is the British spelling whereas "color" is the American spelling. (Leave it to the British to add or use an extra vowel which serves no apparent purpose!) But other words, such as the word "gray," aren't as immediately obvious to me. Consequently, I've sometimes found myself vacillating between the two spellings, depending on my mood.
Here's a link to a useful Wikipedia article regarding such spelling variants. According to that article, "gray" is the American spelling and "grey" is the British spelling. I'd kind of thought that that was the case, but I couldn't recall for sure until I checked it out just now.
I'm recording this information here mostly for my own reference, but it may also be useful to people other than myself.