"This boy's coat is colorful."
Translation:Płaszcz tego chłopca jest kolorowy.
It does not work well. I am not 100% whether it is weird but acceptable grammatically or just incorrect .
Usually we put "owner" after subject. (when it is noun not pronoun)
boy's coat= zeszyt chłopca
Kasia's sister= siostra Kasi
my mom= moja mama
While grammatically correct, it sounds unnatural, as if you were writing poetry and trying to fight rhythm or rhyme constraints. But people generally don't talk like that.
The translation just does't sound right. Sounds better the way Kulink had it in his comments
Shouldn't "Płaszcz tego chłopaka jest kolorowy" be accepted too? chłopaka = boy in Genetive.
True, English is a bit ambigous here, although I think that would probably be: boys' coat (a coat for boys) which in Polish we would translate as "chłopięcy płaszcz" or "płaszcz dla chłopca/chłopców".
We also can think that the boy has several coats and we are talking about the certain one