Thank you very much @lemouth :-)
You have very good eyes :-) I'm living here under a satellite and plane racetrack. Fortunately, the satellite tracks mostly get out of the image after stacking.
I'm using a color camera, it has a regular DSLR CMOS sensor, for the QHY247C it should be the sensor that is also in a Nikon D5300. For this image, I used a duo narrowband filter, which lets through the emission lines of H-alpha (red) and OIII (blue). Green is a little bit of a mix of those colors. I thought about buying a mono camera before (where you get the color with various filters and would have a better resolution due to the lack of the Bayer matrix), but I thought frustration could be high when I would only be able to get 1 or 2 colors and then the weather turns bad for weeks.

That's no naive question :-)
There are several reasons for this. The first one is, that it was a little bit of misty/hazy so the light of the streetlamps gives the water particles in the air a nice "glow", which gets this color through the H-alpha & OIII filter. Also without wet air, I get some weird foreground/background colors and gradients due to the surrounding light sources. The second one is, that the Bayer matrix on the sensor has 2 times more green pixels than red and blue ones. So the pictures shot with a color camera will have a greenish tint.
It's no problem to remove it in editing. It's just a color calibration thing like the white balance on normal daylight photography. Gradients caused by light pollution can also be removed in editing, even if it is a little bit harder to get rid of them.