sniffs I-it really is
"Yellow" as a noun should (could?) be "노랑색".
노랑 - noun,
노랗다 - desc. verb,
노란 - adjective.
No need to add anything
What's wrong with "노란색이 예쁩니다" ?
Nothing. You should flag it. The Duolingo machine is currently not capable of checking the complete list of valid translations and relies on manual submission to update its list.
I think both might be acceptable? Not sure.