Says who? This StackExchange is for people writing fiction. They can do whatever the hell they want.
It's fine to advise against certain things that might detract from credibility, but telling people they "cannot" do things comes across awfully self-important.
EDIT: Maybe I'm getting downvoted because of the self-importance of telling people they cannot tell people that they cannot do things? :)
Worldbuilding.SE is weird. People are very unclear whether they want "plausible fiction" or "hard science" answers for things, and to some extent it's also treated like a puzzle - which I think is what the phrasing means here. "You may/may not" phrasing appears in exam questions.
This hasn't been my experience. Worldbuilding has hard-science and soft-science tags, and the better posts use them. Puzzle questions tend to get flagged and closed.
It's the asker saying "you cannot," likely because of decisions he or she has already made about the setting and characters of the story that the green star will be in.
Your response would only make sense if it were an answerer trying to tell the asker what he or she cannot do.
I thought this was a link to an answer, and I thought the question was merely "What could make a star green?"
Starting out with a bold "stars are never green" and then explaining why made it seem to me more like an answer than a question, and the brief questions seemed rhetorical.
Says who? This StackExchange is for people writing fiction. They can do whatever the hell they want.
It's fine to advise against certain things that might detract from credibility, but telling people they "cannot" do things comes across awfully self-important.
EDIT: Maybe I'm getting downvoted because of the self-importance of telling people they cannot tell people that they cannot do things? :)