For the separate material route, you can make an array of material references in your MainMenu BP. When you press a button, set an integer to the index of the material you want and ‘get a copy’ from the array. That would get a copy of the material you could use on the tunnel mesh. You can also save the index integer in your save game and use that to set the material when you load it.
I’m not at a computer right now so if this doesn’t make sense I can try to make a mock up of it in UE4.