A:Answer First, I just want to mention that if the screen looks white with the projector off, then the screen is installed upside down. It should look more gray with the projector off.
In my opinion, the screen actually looks similar to a flat screen TV when turned off due to the gray tone.
If you want to add some interest to the room, you can put some art (or fish tank) on the screen when idle. But I prefer to leave it off when not in use to conserve power and the projector.