Today the skyline of New York City is defined as much by what is missing as by what remains. After the attacks of September 11, 2001, the lights on the Empire State Building stopped changing colors for a year, and a simple tribute of red, white, and blue illuminated what was once again the tallest building in the city.

This Labor Day weekend, through Monday, red, white, and blue will shine again over Fifth Avenue and 34th Street. And in 10 days, the red, white, and blue lights will return, marking the fifth anniversary of the terrorist attacks.

The lighting of the Empire State Building has been a work in progress for 50 years, defined as much by imagination as by technology. It is hard for us to remember a time when the building was not aglow in the darkness above the city that never sleeps. Yet it was not until May 3, 1956 — a quarter-century after the construction of the building — that New York State Lieutenant Governor George B. DeLuca pressed a button that turned on four beacon lights, mounted on the 90th floor, that lit the Empire State Building like a chandelier in the sky.

The building’s management had first conceived the idea in the 1940s. If the shining torch of the Statue of Liberty in New York Harbor was welcoming visitors by boat to New York, then the Empire State Building would greet the increasing number of visitors arriving by air.

“We are indeed proud to become the assistant to the good lady at the Statue of Liberty,” a director of the Empire State Building Corporation, Lester Crown, said at the lighting ceremony in 1956, “and we are hoping that we will glow together with the spirit of freedom for many generations to come.”

Colored lighting was added in 1976. To celebrate the Bicentennial that year, the top of the building was bathed in red, white, and blue.

The original four beacon lights have since been replaced by a more contemporary and energy-efficient lighting system. Today, 12-foot outdoor landings house 106 lights on the 72nd floor, 72 lights on the 81st floor, and 24 lights on the 89th floor. Encircling the edges of the building and pointing skyward, the barrel-like drum fixtures each contain a 1000-watt bulb. They are bolted down to the concrete, which on floors 72 and 81 is littered with dried-up chewing gum delivered from the observation deck above.

To change the color of the lights, 24-inch-diameter Lexan discs, a quarter-inch thick, are placed over the drum fixtures and secured with four wing-nut fasteners.The vertical lights — four sets of 11 fluorescent fixtures — running up the mooring mast of the building require a less complicated procedure: A switch on the 88th floor changes their colors.

An astronomical clock causes all of the lights to turn on 45 minutes prior to sunset. At midnight, the colored lighting shuts down, and 32 70-watt high-pressure sodium vapor lights ring the top of the mooring mast like a halo above the building.

In the past the building’s team of six electricians changed the colors only on major holidays. But today, the colors change 70 to 80 times a year, recognizing not just holidays but charities, film shoots, seasonal changes, and a host of other occasions. The building can provide five different colors — red, green, blue, yellow, and white — in different combinations for different effects. (Just this past month, green/white/orange, from bottom to top, for India Independence; green/green/white, for Pakistan Independence; and yellow for the U.S. Open.) A total color change takes nearly four hours to be completed by the experienced crew.

To commemorate the death of President Ronald Reagan in June 2003, the building was dark for two days.

One of the electricians, Louis Esposito, 51, has been part of the team since 1980, when he began as an electrician’s helper for $7.10 an hour. His favorite view from the top of the city is northeast, toward Long Island Sound. “From here you can lose perspective,” he said. “It’s like you could reach out and touch the surrounding buildings.” The first time he worked on a light change at the building he said, “was scary, awesome, and impressive.”

Today Mr. Esposito has a different view: “This job is sort of peaceful, in a way. Especially when the world is sort of hectic down there and you can kind of get away,” he said. “Any color change is a statement and it unifies the city. It’s patriotic, and people feel better about who they are. If it can do that, help a person feel patriotic, then I guess it is a good thing.”

Mr. Coll is a frequent contributor to The New York Sun and an adjunct professor of history at Nassau and Suffolk Community Colleges.