CarlG is correct. You can use the Z to separate that part and apply it with another material.
here’s a good example:
It’s pretty simple and easy to make an adjustment.
I was using Blender 2.82.7 and it should be possible in 2.83 without any problem. Just keep this in mind, 2.83 is still in Alpha.
Hope this helped!
I added one more materials to the object so you can have 3 different materials on one object. and The object I forgot to mention was that it also has the custom profile with a bevel modifier.
If you want the whole ceiling to just be white… then you can remove the Node setup #B and leave #A alone. You also just need to remove the light blue node and mix shader. then you’ll be all good to go.