Application of materials in obj file problem with visualization in Hierarchy

Hello to everyone
I am trying to import an obj file (ballatoio). I want to apply my materials from Unity.
I created the prefab of this imported model (dragged it on Hierarchy and dragged again in Project folder).
I applied the materials and in preview I can see the change of colors but when I drag it on hierarchy the mesh is grey.

Have you got a solution? Of course I am doing an error.
Thank you