Κόκκινος/η/ο όπως μια/η (ν)τομάτα,
Κόκκινος/η/ο σαν (μια/τη) (ν)τομάτα.
I could accept any combination, but it is impossible for Duolingo to do the same for every sentence. Let's be patient, as its bank gets richer every day.
Both are correct, because they are equally used. We could say "τομάτα" is juuust a bit more right, since the word τομάτα originates from the Nahuatl (aztec language) word tomatl. But both are used. ^.^
Ευχαριστώ! When I began learning Greek, people asked me if I was learning Ancient or Modern. Thought it was like saying, "Are you learning ancient (or contemporary) French, Spanish, Portuguese, Italian"?