Layer Masking out shadow in real estate photo leaves behind faint discoloration in luminosity mode

With the ambient shot in luminosity mode, the color component is entirely from the flash exposure. It is not unreasonable to expect a color shift from deep shadow to highlights. Reflections off nearby surfaces will do that.

So the why is no mystery.

In cases like this, I usually put a steep Curves layer on top, to mercilessly exaggerate all problems. Then it's often easier to see the best way to fix it. Maybe painting on a separate layer above the flash exposure is enough - you can set it to Color mode, but as long as the top layer is Luminosity, anything underneath will automatically take on the Color property.

It might be helpful to see your layers, but withtout more information, I would set up some color sampler points, turn off all the layer masks, and isolate each layer in turn while watching the values in the Info channel.  It seems reasonable that the flash lit layers will be closer to the slight blue tint in the rest of the ceiling.  If I find that I would copy the relevant are to the top of the stack, and feather any layer mask to get a soft transition.

The problem AFAICT is that the affected aera is rather irregular, so difficult to select.  Color Range was not doing it.  It's not even easy to paint out, and I don't know if you'r be allowed to use that sort of fix/content in a realestate image.