Humans aren't exactly able to see green or purple stars. Stars send out their radiation in different wavelengths and whichever colour's wavelength peaks, that star is then visible to us in that singular colour.
Humans have evolved to see yellow and somewhat green Radiation because our own star primarily emits those wavelengths but it appears yellow to us because that wavelength peaks.
A green star would probably appear white to us (or a little something like blue) since green is in the middle of the colour spectrum and that star would probably emit a little bit of all colours which will end up as white.
Similarly a purple star would appear to us as blue since our eyes are more sensitive to blue light and purple is very close to blue.
Superman would definitely be able to distinguish between the stars and their colours but the way our eyes work, we are only limited to seeing red, yellow, blue and white stars.
