"Grey" is just as acceptable as "gray". It's more common in British and Canadian English, I believe, although none of the dictionaries I've checked even specify "grey" as a chiefly British form of the word, just an alternative spelling. As a matter of fact, I personally prefer the spelling "grey" to "gray".

Speaking of British/American differences:

Originally posted by wikipeeeeedia on doubled consonants

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The British English doubling is required for all inflections (-ed, -ing, -er, -est) and for the noun suffixes -er and -or. Therefore, the British English counsellor, cruellest, modelling, quarrelled, signalling, traveller, and travelling. Americans usually use counselor, cruelest, modeling, quarreled, signaling, traveler, traveling, etc.

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I have a tendency to use the British English variants of all of these words if I don't watch myself. They look more natural to me, but probably not to most Americans.
If not for the fact that I'd probably seem like a complete weirdo, I'd probably be using "colour", "favourite", "judgement", etc. I've had to train myself to write like an actual American and not a Canadian, I swear.

Also, "separate", similar to Terra.

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Whenever I'm unsure of a word's correct spelling, I head over to dictionary.com to find out the proper.
"Mozzarella" is the only word I can recall (awful memory) that took some time to memorize.

Any words I misspell out of ignorance of the correct form are usually quickly corrected by a ~certain person that I know~. Since my grammar/orthography are of very few things I have passion for in terms of knowledge, I make an effort to learn the correctness of my errors the second they're made apparent to me.

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Exate Land - ZC/ZZT works from years past.