"The spider and the cat are gray."
Translation:Η αράχνη και η γάτα είναι γκρι.
Και is also correct. It's not necessary to change it to κι before a vowel, but of course in spoken Greek it's usually 'κι' in those cases, as you say. I don't know if κι has been added as an answer in all (or any?) of the sentences where it may appear, but I would think not because it is too much to expect all spoken variations to be included in Duo's system (same with all the vowel omissions and apostrophe phenomena). Sticking with the formal language pays off here! :D