"These are the cats whose color is brown."
Translation:Αυτές είναι οι γάτες των οποίων το χρώμα είναι καφέ.
- your accent marks are missing *
- the relative pronoun οποίου is missing the definite article (the forms are not of οποίος but of ο οποίος)
- you used the masculine/neuter singular (του) οποίου to refer to οι γάτες which is feminine plural
- the noun χρώμα is missing the default article -- more literally, you need a translation along the lines of "of which the colour is brown"; just "of which colour is brown", without article on "colour", does not work since this is a countable noun in the singular.
* if you're using a physical keyboard and don't know how to make accented vowels, try pressing the key to the right of the L key and then a vowel key.
Normally for one-syllable words you don't put a stress mark because there's no need to indicate which syllable is stressed.
The main exception is when the word is a homonym like πως/πώς, in which case I believe if one of the homonyms is an interrogative word, you put the stress mark on that. That exception doesn't apply here.
I think in this case there's no ambiguity since των has no homophones?
I think the list of accented monosyllables is limited to ή, πού, πώς.
My old grammar also included τί but I've never seen that in use. (And unaccented τι -- a short form of γιατί in the meaning "because" -- seems to be limited to poetry anyway, so in practice, τι will always be the interrogative and you don't have to mark it.)
Plus the "indirect object" pronouns μού σού τού τής μάς σάς τούς when they could be confused with possessives (η δασκάλα μού είπε "the teacher told me" versus η δασκάλα μου είπε "my teacher said").
Yes, it's the word for male cats. More often than not, the Duolingo grading system shows answers (from the pool of answers we have entered) to the user that are closer to their (spelling) mistakes, rather than show the correct answer that's made up of words the user has already encountered. Maybe that's why it confused you.