How to apply different materials to imported structure?

I am a Novice at UE4, I just imported a Revit file in to unreal and would like apply different materials. when I drag a material onto a wall, it changes everything that I imported to that color. whenever my mouse goes over the structure an orange line appears over the perimeter of it, implying that my multi story building with multiple rooms is one mass, do I need to explode the structure some how?