How to create a closed building world and change floor color?

I cannot find answer to these basic questions anywhere. I see people create maps like scary horror genres in closed basement or something. I want to do something like that, just have a ceiling, walls, floor etc. Also how can I apply custom floor? Or hange color of existing floors?

You can use materials to texture different assets such as cubes or other shapes you place with the modeling tools in UEFN or fbx models you import from other 3d applications. Materials can be made up of textures you import, colors, patterns, as well as different attributes you set in the material editor.

I would definitely read up on materials in UEFN to help: