A coat of matt varnish and the whole thing is darker with more contrast again … oh, well.
I then removed the masking tape, during which I managed to push one in:
It took a lot of fiddling with tweezers and a sculpting tool to get it back in place, during which I had to hold the model above my head so that the plate the window sits in, was more or less horizontal.
Also, it turns out that putting tape on the front windows before glueing them in was not the best way to do it. Some of the tape gets caught between the window and the frame this way, so anyone else building one, I would advise to only tape off these windows after glueing them in place. There is no such problem with the portholes in the sides, though.