The Empire State building, arguably New York’s most famous Art Deco structure, has used colorful light as a sort of bulletin board since 1976, when, in honor of the nation’s bicentennial, it was lit up in red, white, and blue. These days, the skyscraper features special lighting schemes for any number of occasions: red and green for Christmas, rainbow colors for Pride Week, purple and white for New York University’s graduation day, green for Earth day.

“It becomes a message board for the city, where the colors start to mean something,” said Jennifer Bonner, an architect and an assistant professor at the Harvard University Graduate School of Design. Just as a flag at half staff can signify mourning, a skyscraper lit in a certain color scheme can represent celebration, advocacy, or grief. After the terrorist attacks in Paris last year, for example, dozens of buildings and monuments were lit in blue, white, and red.

“In a way, these lights being applied to buildings are creating new flags,” Bonner said. “People didn’t literally go out and get [French] flags; we did that electronically, at multiple buildings throughout the U.S. and the world.”

The Empire State Building was not one of them. Instead, it joined the Eiffel Tower in turning off its lights altogether in the days after the attacks. Other buildings in New York lit up instead. Both the spire atop the new World Trade Center and the arch in Washington Square Park were illuminated in the colors of the French flag—symbolic choices, since the World Trade Center is one of the sites of the 2001 terrorist attacks on the United States, and the arch was modeled on Paris’s Arc de Triomphe.

As LED lights have become cheaper, more buildings are adding lighting technology that enables colorful, customizable displays. But not all of the buildings that lit up to honor Paris were already outfitted with such technology. The arch in Greenwich Village, for example, had to rush in a lighting expert for its blue, white, and red display. After two nights, the equipment was dismantled and the arch returned to its standard white lighting.

And though the lights can be automated, such displays still require people to decide which colors to use, and in which combinations, and when. “If color is programmed without a lighting designer, then in all likelihood it will run on default programs set by the factory,” said Linnaea Tillett, a lighting designer and the founder of Tillett Lighting Design & Consulting in New York. “It is simply too complex to be set by a non-professional.”

Color is a complicating factor today. Long ago, communicating a message in light meant simply choosing between on and off—or maybe how many lights to use. In Boston, in 1775, it was a message written in light that set off Paul Revere’s historic ride. The famous line, “one if by land, and two if by sea,” refers to the number of lanterns that were to be used to signal the route of incoming British troops. Today’s Bostonians are taught to recite another few lines of verse to help decode the light message broadcast from the weather beacon atop the Old John Hancock Building: