Various English teachers of mine at both high school and college levels have all said "G-R-E-Y is the English/European/non-USA standard spelling, while G-R-A-Y is the American standard."

So "grey" is JDM/EDM ...lol

I go with FatBooty on this: interchangable. Just stick with one when you've used it, don't use both in the same paper or anything. Be consistant.