[JMStewy]

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.

[jstanley]

Ah, I see! I understand now.

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.