"주황색인 낙엽이 예뻐요."
Translation:The orange fallen leaves are pretty.
I think it has to do with the particle on "leaves".
If the statement was general (so without "the" in English), I would use "는". But here, we use "이", which I think is a sign of talking about specific orange leaves. And in that case, it's more natural to say "the orange leaves". To me, this is the answer to a question like "which of these leaves are pretty?" - "the orange leaves are pretty". Or maybe someone complains abouf the muddy brown leaves on the floor, and this person says: "(well), the orange leaves are pretty".