"Un vestido marrón."
Translation:A brown dress.
Not sure if it helps, but it just is in español. most o ending words are masculine and a ending words are femenine.
Of course, plenty of exceptions.
Cafe is "coffee". But this may help; I associate maroon with a reddish brown. So when I see marron, I think brown.
I am going to prom in a brown dress. Support me by going on duolingo tomorrow too!