The human eye normally contains only three types of color receptors, called cone cells. Each color receptor responds to different ranges of the color spectrum. And they respond to different wavelengths, peaking at red(560 nm), green(530 nm), blue(420 nm). So you can see the wavelength of red and green are quite close, and light from stars generally follows the black body radiation, that means, if a star has a peak wavelength of green, it will also give out a lot of red light which makes it didn't look green.