You can use PostZone(Post Processing)Component to add/change postFX in your region. PostZone can be applied to a part of the region or the entire region.
To create a PostZone, create an empty object in your scene, then click on add component, search and select PostZone.
If you want part of a region to have different postFX, then add a box collider to the EmptyObject with PostZone and size according to the area needed.
For example you can have different postFX for interior and exterior areas of a region by adding different postZone and sizing the collider to the size of the area.
PostZone Technical Details
You can learn more technical Details on Post Processing here.