Plan showing how the red wall was generated - what is in red is not seen from the entrance.

When seen from the far doors, the red walls are invisible

When you are in the corridor, you see what is not visible from the door.

Detail of what the eye would not see.

Peculiar shadows are created where conduits and pipes obscure the wall.

A switch at the end of the hall, the red is what you do not see from the entrance.