To take advantage of my self-imposed unemployment the wife and I have been vacationing it up lately. In February 2013 we took a cruise on the Disney Magic to Grand Cayman and Cozumel out of Galveston. In your state room you are given a door hanger that has two very different messages on it. This is to let your room host (the person responsible for servicing your room) that you either don't want to be disturbed or that you'd like for them to attend to your state room. Here's what they look like:
A couple of cruises ago we spent almost half the cruise not realizing the two sides were different. Around the third or fourth time the host tried to service the room while we were taking a mid day nap we realized what idiots we are. That got me to thinking about how you could improve these door hangers both for the guests and for the room host. For the guest you want something that makes it even more obvious that the two sides have different meanings. For the host you would like a method of differentiating the message being communicated by the door hanger that can be discerned from a greater distance or from a more extreme angle that doesn't allow reading it. Of course you have the different images on the hanger but that's not terribly visible when looking down a hallway of a couple of hundred doors.
I think using a different color for each side would work even better. Since we're not communicating the intent of the different sides solely through color I think were still good in terms of accessibility and color blindness (I should also note the lack of Braille on the different sides of the door hanger). I would naturally pick red and green if not for the horrible memory of a former manager and a firmware engineer arguing for hours about the inherent ambiguity of the two colors when multiple nationalities and cultures were involved. Anyway, here's a stab at what my "improved" version would look like.