It is important to remember the verb είμαι (Ι am). So:
είστε or είσαστε
Imperfect ( in the past)
ήσαστε, or sometimes, ήσασταν
There are more tenses, Future, Future Continuous, and Subjunctive, but it is enough for the moment.
Both spellings are correct: grey is favoured in the UK while gray is more common in the US.
The word "grey" was added about 2 years ago, so it is considered correct. We can't do much unless you send us a screenshot of your answer, though.
why isn't it γκρίζα? Is this because when you describe something in the predicate, you use the name of the color itself?