Green, orange, blue, or red strars, what are they?

You answered your own question when you posted the subject line, actually; the red, orange, and blue stars you see are just that – stars. And the color of a star has a lot to do with what temperature the star is.

Small, hot stars like Bellatrix have surface temperatures of around 27,000 degrees Kelvin and give off a rich blue light. Extremely large, (relatively) cool stars like Betelgeuse burn at around 3,000 degrees Kelvin and show as deep red.

Our own sun, Sol, is a fairly cool star as far as stars go, with a surface temperature of around 6,000 degrees Kelvin, and as you are probably aware, this results in a yellow-white light.

There are other forces at work, too, that change the light we can see from the stars. Elements in the stars’ own atmospheres are absorbing wavelengths of the starlight, as are the elements in our own atmosphere. Also, a star’s light may be red-shifted or blue-shifted depending on if it is moving towards us or away from us thanks to the Doppler effect. But in general, if someone asks you this question and you say “Cool stars burn red and hot stars burn blue,” you won’t be wrong.