How I instantiate specific decals to specific textures?

For example our mesh has 4 materials, wood, metal, brick and plaster. Now how do I instantiate their respective bullet hit marks?

I have one temporary solution, overlay a plane on each with tags named after the surface it is laid on.

I cannot use tags and layers since it is only one mesh like a building for example.

I have one idea like using rgb vertex colors but I do not know how to implement it. I have vertex painted the mesh.

Any help is appreciated

 

How are the materials mapped to the mesh?
Just through UV coordinates? If so it might be tricky, but still possible.

Make a tiny texture that's (for example) pure red for metal, pure blue for brick, ...
On hit look up the UV coords (should be given to you by a raycast automatically already), then just look at the pixel color of your lookup texture and you know what material type was hit.

Remember to save the texture uncompressed