Translation:Xanh da trời
Because vietnamese has technically only one word for both blue and green, which is xanh. The language just doesn't distunguish colors blue and and green per se. So if you really wanna make the difference (like duolingo wants) whether you are talking about blue or green, you add something describing that color.
Xanh da trời means literally sky grue (or sky blue), Xanh dương means literally ocean blue Xanh lá cây means leaf green.
More information about blue/green distinguishion: https://en.m.wikipedia.org/wiki/Blue%E2%80%93green_distinction_in_language
ugh, this is so frustrating... because in southern vietnam, we only say "mau xanh" for blue... and if it's green, we say "mau xanh la cay" = literally means green/blue color of tree leaves... Idk, but that's what I grew up with. I never heard of xanh duong or xanh da troi in my life.