The color of the day is a signal used by undercover officers of some big city police departments in the USA. It is used to help stop uniformed police officers shooting undercover police officers who they do not know are police officers. It is mostly known to be used by the New York City Police Department (NYPD), the NYPD Transit Bureau (which was called the "New York City Transit Police") and the NYPD Housing Bureau (which was called the "New York City Housing Authority Police Department") and other law enforcement agencies when they are at work in New York City. An undercover police officer will wear a headband, wristband or other piece of clothing in the same color as the "color of the day"; this color is told to officers at the precinct before they start work. The system is for officer safety and first started during the violence of 1970s and 1980s New York City. The color of the day system is not known by most of the public but it is written about a lot by newspapers, and by writers and can sometimes be seen on episodes of television shows such as NYPD Blue and recently .