The colours of stars are :- red, orange, yellow, white and blue plus there's variations of those colours. There's no green even though green is a colour in the rainbow. The colour of a star gives you an indication how hot the star. The redder a star is, the colder the star is and the bluer the star, the hotter the star is.
A star will emit visible radiation and the colour is as above dependent on the temperature. Green appears in the middle of the colour spectrum. A star emits a range of colours rather than just one. When a star is emitting in red, it does so because its at the very edge of the colour spectrum and there is not much influence of other colours so it appears red.
Its the same with blue, its at the edge of the colour spectrum and therefore there is not much influence by other colours. As green is in the middle, other colours on either side of the spectrum collude to make the star white. A notable white coloured star is our own, the Sun but it appears as yellow in the sky and white outside the atmosphere.
It is quite feasible to say that white stars are really green stars but because of the spectrum of colours a star emits, the green is merged with other colours to appear white.
What about purple stars, some stars might emit very hot and therefore should appear purple. As stars will also emit blue as well as purple, our eyes have evolved to pick up blue better than purple. Therefore some of those stars out there such as Regulus and Rigel may well be purple stars but we can't detect them due to evolution.
The star Zubeneschamali in the constellation of Libra is a star that has often been quoted as being a green star. It is officially classed as a blue or white star. Some people have said to have observed the star as a green star such as William Tyler Olcott who described as it as being the only true green star out there. Olcott founded the American Association of Variable Star Observers (AAVSO.ORG).
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