You could do a white / off white (dependent on other colours) for the arch, and just have the actual colour only go to the arch edge on each side perhaps? It can look quite well, particularly if you are using a strong colour on the rooms.
I did wonder that, I was going to do the ceilings an off white, so maybe like a stripe of that then on the inside of the arch? I suppose if I tape it off and make sure it's neat it could look good. Thanks.