The grey of a hotel door hanger that means leave the guest alone.
Do Not Disturb is a dark grey.
It can be described as dark, dim, somber and gloomy.
Set in white, because white reads best on it.
A door hanger is a plastic or cardboard sign, generally rectangular in shape, cut to hang from the handle or knob of a door. They are sometimes used to distribute print advertising to residences. Door hangers are often seen in hotels and other places of lodging as a means for guests to communicate with maintenance and housekeeping staff.
Read on WikipediaThe Do Not Disturb or (DND) function on most PBX or PABX systems prevents calls from ringing on an extension for which DND is activated. Some Do Not Disturb (DND) attributes include directing the call to a preassigned extension, busy signal, DND signal, or recorded message generated by the telephone switch. Some switches allow the call to go through to the extension but restrict the response to a visual indication. Some PBX systems allow the assignment of DND circumvention codes to supervisors. The implementation of this feature is vendor-specific. In some cases, the ringer just does not ring and the called party is thus not alerted. Usually, though, the phone just acts as being busy. Depending on the infrastructure, the caller may end up on the called party's voice mailbox, sometimes after a certain delay. Some special configurations combine DND with other features like call forwarding, where calls to a party with DND on are forwarded to another phone.
Read on WikipediaDo Not Disturb: The Story of a Political Murder and an African Regime Gone Bad is a 2021 book by British journalist Michela Wrong, published by Fourth Estate and PublicAffairs. The book focuses on the 2014 murder of Rwandan defector Patrick Karegeya, for which the Rwandan government denied responsibility.
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