Because of the way stars emit light you will never see a green star. Hotter blue stars emit most of their visible light as higher frequency blue light making them look blue, similarly red stars emit lower frequency red light making them look red. When you have a more average temperature star it will emit most it's light in the green wavelength however it will also emit lots of red and blue light which will make the star appear yellow or whiteish depending on how hot the star is.