What I was wondering is if I can change the colour of light that glowing objects emit, e.g. lamps or the neon bar, with a recolour. (It seems to be possible since different recolours of some lamps have a different coloured light, too. But if I create a normal recolour, does it contain the things I'd need to change for a different light? My guess would be the material definition, but idk...)

I tried googling, but it's a bit hard to find anything because there's a lot of different things it spits out for the keyword "light". So if anyone knows about a tutorial to help me out here, a link would be great.

The light emitted is usually defined in lights.txt in program files.

So what happens if I recolour a light with different colour options?
Let's say this long wall lamp. It emits a red light if you choose the red recolour, etc. Now if I recolour that and choose the red version when creating the package, is it just gonna collect the texture files of that one or will it actually shine red, too?

There is a light mod available on here somewhere that changes the nature of the in-game lights - you might learn something from that. I'll see if I can find it.

Here - it is plasticbox:
https://www.modthesims.info/d/325577

There's some info about what she did and, as she's still very much around, you might try and talk to her. The nature of the colour though appears to be linked to the mesh.

Uff, okay. I just tried it out, and one wall lamp looks pretty much normal, but this table lamp I did doesn't emit any light at all when choosing my recolour. (Just this "painted on" light that some have, where you see its shape in the textures when recolouring and that is thrown on wherever you place it. But no object around it is lit.)

So I guess I'm having a look at that post from plasticbox.

Strange...