Green stars are normal with certain nebulae filters! Here’s why:

Many broadband nebula filters let through light at the wavelengths of Hydrogen Alpha (Hα), the Oiii doublet at ~5000Å, and Hydrogen Beta (very close to the Oiii doublet). So you have a passband at ~5000Å and one at 6563Å. 5000Å is green light, while 6563Å is red light.

1.) If a star has a moderately high or high surface temperature (F,A,B, and O types, at least) it should be brighter at the wavelength of 5000Å than at 6563Å, due to its blackbody curve.

2.) Those types of stars often have strong Hα absorption in their spectra, but no Oiii absorption, so the light being passed by the red part of the filter has been dimmed even more by the star’s own atmosphere.)

 

3.) On top of that, some filters have a wider passband at 5000Å than they do at 6563Å, so even if the star’s spectrum was equal at every wavelength, more light would still get through the filter at the 5000Å area!

You also need to take into account the spectral response of the camera itself: does 5000Å fall into the area covered by the green pixels or the blue pixels? It looks like for the average DSLR, 5000Å is right in the transition zone– so you should get a blue-green color when you image 5000Å light. And if your DSLR is unmodified, Hα light at 6563Å is probably being suppressed by the IR filter above the sensor, further skewing the balance towards blue-green. You can look up the spectral response curve (or Q.E.) for your particular camera, if you want to see where these wavelengths are landing.

I overlaid the transmittance curve for the Optolong L-eNhance filter over a spectrum I took of one of the components of Kuma (an A class star), to illustrate what I’m talking about– it’s way nicer than trying to visualize the situation from just reading numbers. I hope you find it as nifty as I do! 😉

In conclusion: I wouldn’t worry about your green stars. I’m sure there’s some processing technique out there to get rid of them, if they really bother you, but I don’t think it indicates a problem, and green is likely exactly how they’re supposed to appear.