Yellow stains on a white wall, should they be covered with white paint?

Good day, I hope you can help me. My girlfriend and I recently moved into our first shared apartment. Since neither of us are big fans of yellow poorly wallpapered woodchip wallpaper, we tore it down and her brother, who is a mason, plastered the wall with fine plaster. Now we come to the annoyance. In some places, yellow stains are seeping through the wall, but these are basically only where there are wooden beams underneath. My brother-in-law suspects that the previous tenants smoked heavily in the living room (the problem is only here) and that it comes from that, but none of us are sure. Is there a way to permanently hide these stains under white paint? The house is an old building. We still have some renovation work ahead of us and would therefore like to save unnecessary steps if possible so as not to be "pointlessly" held up further in a room.

Hello Cutepie !!!
Well, I can't really say what kind of stains these exactly are, maybe you could chisel them off, but whether that's still sensible once everything is finished, you have to decide.
You could isolate the remaining stains, one option would be to spray them with stain blocker (isolate) or the second option is to chisel off the plaster and replaster.
Best regards.

There is a so-called blocking primer at the hardware store that prevents the bleeding of nicotine, etc. It is usually white - but can also be painted over without any problems. This should solve the problem.

Hi Joerg!!
A tip on the side!
Your so-called Sperrgrund from the hardware store is this stain blocker I mentioned, and it is also available in spray cans.
Best regards.

Sorry, I didn't know that - I haven't seen that in spray cans yet and I had something different in mind under Fleckenblocker. I only know the stuff as a primer for problematic substrates.

Hello Joerg !!
Doesn't matter, it wasn't meant badly, just for your information
Best regards.