By default, face fronts are white and face backs are light blue, although you can change these colors. Face fronts should be exposed to view; face backs should be turned away from the camera. In the case of a closed (solid) shape, such as a cube, it should appear white all over. All the blue faces are turned toward the inside of the cube.
You can reverse faces with the like-named command on the face context menu (right-click > Reverse Faces. You may choose to turn all other faces to the same orientation as a selected face by using right-click > Orient Faces.
Avoid painting your models with a texture or solid color until you are satisfied with the basic geometry and no longer need to refer to the face front and back colors. If you do paint an object, you may still see the face colors by using the Monochrome style setting.
Four walls would be like a cube-in-a-cube. The space between the faces is the “inside” of the wall. So turn all blue faces toward the inside (unseen side) of the wall, leaving all the visible faces, both inside and outside of the room, white.