Pick the room you wish to customise and use the Roller icon to enter the customisation menu. Here you can pick any colour or material for walls (both inside and outside), floor and ceiling.
Pick colours and materials you like to apply them to your project.
In case you're using another platform (iOS, macOS, Android, Windows) you will need to tap on a room and use the Edit option that'll appear.
Don't forget to check out how that looks in 3D mode!