There are roughly 8.25 million residents of New York City. Protecting and serving those millions are about 35,000 to 40,000 police officers, not including traffic enforcement agents, school safety agents, auxiliary police, and others. Of those 35,000 or so officers, most of them wear uniforms to work each day. But some of New York’s Finest dress in civilian attire — undercover cops, hoping to blend in. If the officer is successful, no one knows he or she is a member of the police department.

No one — including other officers.

As one can deduce, this can cause a problem. What if, during the commission of a violent crime, uniformed police officers attempt to subdue and arrest the group that the undercover officer is with? In the worst case scenario, the uniformed officers could end up firing at the alleged criminals — and, in turn, at their fellow (undercover) officers.

This problem is by no means fiction. On August 22, 1994, as reported by the New York Times, two teenagers were seen entering a subway station, armed with shotguns. An off-duty officer (in plain clothes) named Peter Del-Debbio fired at who he believed to be one of the gunmen, a man named Desmond Robinson. But Robinson, it turned out, was an undercover transit officer in the pickpocket squad, and he, too, was in pursuit of the same two teens Del-Debbio was after. Del-Debbio ended up firing five bullets at Robinson, striking him in the back with at least two of them, and rendering Robinson permanently disabled. (Del-Debbio would later be convicted of second degree assault for using excessive force and sentenced to five years’ probation; Robinson, unable to return to duty, won a $3 million judgment against the City.)

In hopes of avoiding this, the NYPD had a system in place since the 1970s — one which was unfortunately not followed in the Del-Debbio/Robinson situation — which aimed to signal officers that the “perpetrator” they thought they were trailing was, in fact, an ally. In another article about the subway shootout, the Times reported on that system: the NYPD’s “color of the day.” Each day, the police department would designate a color, and undercover officers were to wear a headband, wristband, or other easily-noticeable article of clothing using that color. (In 1994, the transit police were not part of the NYPD, perhaps explaining why Robinson was not wearing the color of the day.) While this system is not fool-proof — it can cause false positives and does not truly meet the needs of those officers who must make split-second decisions — it is certainly better than nothing.

The “color of the day” system is generally not well known nor often discussed by the police department’s leadership, for obvious reasons. For all we know, especially with changes in technology over the last two decades, the system may no longer be in effect. But either way, it most likely saved a number of officers’ lives.

Bonus fact : The NYPD’s annual budget is $3.9 billion. For comparison’s sake, that’s about 50% more than the market cap of AOL ($2.4 billion).

From the Archives: Barely Legal : A uniform that you probably won’t see an officer wearing — even if it is legal in New York.

Related: A New York City Transit Police Department patch. They are no longer in use and are only a collectible.