I am trying to assign a decal on a wall with 4 type of brick variants, every brick has its own material.
Is there a way I can assign a material on top of the main wall with a decal?
I tried to do it but the material get override everytime.

Thanks

Only one material can be bound to geometry.

If you need layering, then you will need to create a new Material primitive that performs that layering (of shaders).

Because it's an instanced geometry there is no way to assign a material to it as a whole. It doesn't have UV.