Because in english that is a different question ...
I guess the real question is, how would you ask 'what colour is on the shirt' as gaeilge?
No, not really. I have a white shirt. Someone spills ketchup or red paint on it. "What colour is on the shirt?" "Red." How you would differentiate in Irish, though, is another matter. Anybody out there know?
Perhaps Cé dáth an smáil sin atá ar an léine? (“What is the color of that stain on the shirt?”).
"Cén dath atá an léine". What colour is the shirt. "Cén dath atá AR an léine" is on the shirt. Yet on the shirt is wrong
Cén dath atá an léine? doesn't mean "What colour is the shirt?", the ar is a required part of the Irish construction.
Irish isn't just English with Irish words, it uses different grammar and different constructions too.
I did Irish is school, it just struck me as odd from the Irish side, not really including English. I'd have thought without the ar makes perfect sense in Irish, so I was surprised to see the inclusion of "ar" and figured it must want "on" too. I see the mistake