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Worldbuilding

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    $\begingroup$ Most stars look white to the naked eye, since only a very few are bright enough to activate the color-sensitive cones in the eye. And an inexperienced stargazer like the one in the question might not know that green or purple stars don't exist. Maybe if they were all a sickly green, say, that would be eerie enough, but that seems like a fantasy scenario rather than something with a scientific explanation. $\endgroup$
    – Christopher Bennett
    Commented Mar 24 at 22:31
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    $\begingroup$ @ChristopherBennett to be honest, I don't think its possible to have an answer to this that isn't more or less a fantasy scenario. Stars do follow general rules but, to a naked eye observer, most of those cannot be accurately evaluated. So, you kinda have to make something super obvious and super impossible. $\endgroup$
    – Topcode
    Commented Mar 24 at 23:16
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    $\begingroup$ When the observer is on a rogue planet that travels through a galaxy at relativistic speed, then they would see the stars in one direction red-shifted and those in the other direction blue-shifted. That could reduce in unusual star colors without requiring to be in an entirely different universe. $\endgroup$
    – Philipp
    Commented Mar 25 at 10:46
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    $\begingroup$ @ChristopherBennett if I saw a sky full of green stars (or of any other color), my first thought would be that there is some glass or gas cloud or any other transparent substance between me and the stars filtering the light. $\endgroup$
    – phoog
    Commented Mar 25 at 11:46
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    $\begingroup$ @Philipp :-) You have spot a very nice and very surprising point of the physics :-) Fun is that normally doppler makes a red or blue shift. But, the black body spectrum has a very funny feature: if you doppler shift it in any direction, you get yet another black body spectrum, only warmer or colder one! It happens only with the black body spectrum, it has a deep mathematical reason and it has very nice consequences (for example, the cosmic microwave background does not really cool, it is doppler shifting). $\endgroup$
    – Gray Sheep
    Commented Mar 25 at 11:47