What's wrong with carr uaine here?
Strictly speaking, there isn't anything wrong with it -uaine doesn't seem to be part of Duolingo's vocabulary, though.
If the question comes up again for you, you can report it so that it can be added as an acceptable answer in the future.
Both terms refer to the color "of the growing grass". But very few people still use uaine (used in the Constitution). Uaine tends to refer to a vivid, often artificial, green. For most Irish people it sounds dated I suppose...
I like it though...
What's wrong with bhuail an gluaisteán ghlas an balla mór?
I don't think glas should be lenited there (please correct me if I'm wrong) but otherwise it seems like it should be accepted.
@Emma, ghlas would be for a feminine noun.