"This boy's coat is colorful."

Translation: Płaszcz tego chłopca jest kolorowy.

[Kulink]

Tego chłopca płaszcz jest kolorowy, how does this not work?

[immery]

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

but
my mom= moja mama

[vytah]

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.

[ziggy69]

The translation just does't sound right. Sounds better the way Kulink had it in his comments

[DavidConrad1]

Shouldn't "Płaszcz tego chłopaka jest kolorowy" be accepted too? chłopaka = boy in Genetive.

[immery]

yes it should, report.

[MrBTTF]

How could I guess it wasn't "Ten płaszcz chłopca jest kolorowy"?

[Jellei]

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".

[MrBTTF]

We also can think that the boy has several coats and we are talking about the certain one

[Jellei]

Yeah, but I think both Polish and English would rather split it into two sentences, or at least separate clauses.