why is 파다가 푸르러요 wrong? 푸르러요 means both green and blue right?
You write ocean wrong.
바다가 푸르러요 Is correct.
Seriously, I don't know why Duolingo never lets you use 는 after words, like it's never used or something... And it's actually the more popular one...
They, and other educators, should put more effort into explaining the difference between subject and topic particles.