Why is my (white) painted plaster going brown on the borders of the underlying plasterboard?
I had this exact situation when flashing around a chimney leaked and I had some water spots on the ceiling by the fireplace.  The stains from the water can still come through even after the drywall is dry, so you need to apply a sealer and repaint.  I used Kilz; you should be able to find a similar product if that's not available where you live.