"The flag has beautiful color" was rejected, but it seems like this is a legitimate response. The sentence makes perfect sense in English.
I know, right!
Why is the ending of the sentence written with belan koloron and not bela koloro?
Because the beautiful colour is the object of the verb "have" -- direct objects take the accusative case in Esperanto, with ending -n.
Compare English, where we say "I have them" and not "I have they".
Lovely and beautiful can be used interchangeably elsewhere. Why not here?
My complaint exactly. Glad to see that I wasn't the only one dinged for this. But they accept "Nice." Go figure.
The correct answer should use "colour", "color" is not a word
Duolingo uses US English, in which "color" is the correct spelling.