why do we have American English spelling?
Because Duolingo generally uses American English? In fact, that's the one they teach. However, "grey" shouldn't be counted wrong.
Yes I tested this, grey or gray is accepted. Personally not fond of American spellings, but then I guess Duolingo is a programme developed there. And it has taught me more in the last few weeks than I would have believed possible!
I wonder why 'grey' is not banhduhn or something like dat.