This is looking good. We've got a nice foundation to build upon, but it's pretty barren. We can't use textures yet, as it's a ProBuilder-only tool. But we can apply colors to our walls to help differentiate the areas. Let's see how we can do this now.
Go back to our first room and select the top section of our doorway walls. Then, click on the Vertex Colors button from the Prototype tab.
This will bring up an example number of colors, but we can customize them by clicking on the bottom section.
Now, with the faces selected, click on the blue button.
Save the level and start the game.