Stars can't be green for the same reason you can't make iron glow green-hot! It goes from dull red, to white hot, to bluish hot. The thermal blackbody radiation curve that peaks in the green has equal parts blue, green, and red, so it appears kind of yellowish-white to us. If green stars were possible, our own sun would be exactly that! If you think about it, you're glowing like a star right now, just only in the infra-red spectrum, can't see it (unless you have an infrared heat camera) ✨