"I prefer white roses to red ones."
Translation:Je préfère les roses blanches aux roses rouges.
I'd say that is part of the slightly more general rule that one uses the definite article when talking about something in general. So if I said "I eat green cheese" (meaning that green cheese is a thing that I eat sometimes, not that I'm currently eating it) I'd say "je mange le fromage vert" rather than "je mange du fromage vert" (where the latter would suggest that I am eating some green cheese right now).
The rule about preference fits in because when we talk about preference we are talking about particular objects in general.
It seems the second preposition/adjective is "a problem" (aux roses rouges) which cannot stand with the (first) indefinite article (des roses blanches) as far as I understand. At least that. smearedink has already explained it.
Plus he says: "The rule about preference fits in because when we talk about preference we are talking about particular objects in general." Which is, by the way, quite invisible in the English sentence, I'd say.
Why the red roses an acceptable answer. Yes, it can be implied that the red ones are the red roses but the sentence doesn't include roses. Duolingo needs to be consistent in their answers. They take a heart when you erroneously add a "the" that is not there but then also take a heart for not adding a "rose" that is nowhere in the sentence.
• Je préfère des roses blanches à des rouges.
• Je préfère les roses blanches aux roses rouges.
How do we explain the difference in the parallel constructions of these 2 answers? They don't seem arbitrary to me. Googled but found little on "French parallel construction"
The first could be the response to the question, "Which color of roses do you prefer for the place setting?" The second could be the response to the question, "Which color of roses do you prefer, generally speaking?" (And, of course, the second would be required if the English sentence had been concerned with "the" roses).