In Philadelphia, as in other cities, the cranes have left the skyline. It will be years before another high-rise trophy pokes through the clouds, and yet we shouldn't assume that Center City's skyline will just stand still. Peco's newly colorized crown lights are already altering the night-time panorama in ways that are as tangible as any swaggering skyscraper.

Obviously, the scrolling sign on top of the Market Street monolith isn't new. The high-rise message board has been sending out lighted exhortations to "Drive Safely" and "Support the Girl Scouts" for so long that many of us tuned them out ages ago. First lighted in 1971, the crown went verbal with blocky white letters for the Bicentennial in 1976. But the type was so fuzzy, and moved so lethargically, that the effect was like watching someone hunt and peck on an old Remington.

Now, the experience is more akin to catching the latest tweet - that is, if tweets were displayed against the night sky in 40-foot-high capital letters and came with similarly sized photos and graphics.

While Peco's lights blare many of the same banalities as before, it's the medium that was profoundly changed by the July 4 introduction of color and modern LED lights. The messages race around the rim of the 29-story building with the clarity of high-def TV and are accompanied by the sort of peppy animations you might find among the Saturday morning cartoons.

Viewed during the blackness of night, the moving sign seems to energize an an all-too-proper skyline. I love the way the words and images dance a jig across the still backdrop of the lighted Comcast, Bell Atlantic, and Liberty Place towers. Even though the Peco content is often old news, the movement makes the words feel urgent, and the urgency is exciting to look at.

But those incessant waves of color and imagery are less appealing in the early part of the evening, when the skies still have their own internal light. The intense color is at odds with the tower's subdued architecture, a 1970 design by H2L2 that was inspired by the precise modernism of Mies van der Rohe. In the dusky half-light, the Peco sign reveals its true self: a giant electronic billboard that we can't escape.

That's not exactly the Philadelphia aesthetic, is it? This is a town, after all, that quibbles when businesses try to hang small protruding signs over their front doors. Now, suddenly, we have an electronic Colossus standing sentry at the gates to Center City.

Only a year ago, Philadelphia was torn over the prospect of installing a single corporate sign midway up the facade of Two Liberty Place because it was seen as too intrusive and too blatantly commercial. Peco gets away with its high-rise self-promotion because it cloaks it in the public-service message of energy conservation. The company also gives away space to promote nonprofits and their causes.

"We put our energy into the community," the sign claimed the other night, followed by an announcement that it is "World Breastfeeding Week." There were no images with that one.

It's certainly inoffensive stuff. And the promotional aspects are offset by the dynamism of the prancing sign. But what if there were more than one billboard in the sky?

The technology is there and grows more sophisticated by the week. Comcast's lobby display, which is based on a similar LED system, produces images that are even more vividly lifelike than Peco's version. Hang a couple of those screens on a skyscraper, and the skyline might start to resemble the one in the opening scenes of the movie Blade Runner.

In recent years, Philadelphia has been changing a lot of lights. We lost the fairy-tale twinkle of Boathouse Row when it shifted to LEDs to save on energy and maintenance. The Cira Centre tried turning its multifaceted facade into a pegboard of colored LEDs, but the clever concept felt flat because large expanses were left dark. Only Comcast has succeeded in creating a lighting scheme that enhances the architecture.

Peco's crown lights might have been as intense as Comcast's lobby screen if the designers weren't constrained by the crown's other function. Because it also hides the rooftop heating and cooling systems, it was built with a series of vents. In the original lighting design, vertical bands of 30-watt incandescent bulbs alternated with the openings.

To create Peco's new sign, the designers at Cloud Gehshan Associates, a Philadelphia graphics firm specializing in identity branding, had to work around those vents. That's why the images read as if they were striped, rather than a continuous expanse of pixels.

Cloud Gehshan created a series of graphic storyboards for the $1.4 million sign, which were then translated into a computer-controlled system by YESCO, a Salt Lake City company. Because the nonprofits supply their own material, the quality of the graphics can be uneven.

Peco might have left its antique, white incandescent lights in place had the state not mandated its utilities to reduce their own energy usage 40 percent. The sign update was part of a larger $15 million greening effort. Since the tower already had a lighted crown, the colorization project required only a simple electrical permit, according to a company spokesperson.

That's really too little review for something so visible. Colored architectural lighting isn't new; Peco pioneered it when it colorized the top of its previous headquarters on South Ninth Street, near Walnut Street, in the '20s. Moving lights remain as much a symbol of urban vitality today as they did then. But that doesn't mean the public doesn't deserve a say.

If Foxwoods opens its casino on Market Street, launching a new entertainment district, Philadelphia will need to develop standards for the district's lighting and signs. Already, some are suggesting a sign district like New York's Times Square, where we celebrate our gaudy side.

Bright lights don't have to be tacky lights. The only thing separating Peco's electronic billboard from electronic art is, well, the messages. Imagine if the scroll included haiku poems, quotes from Ben Franklin and Shakespeare, or artists' videos. Instead of marking only the usual holidays, how about acknowledging Philadelphia's famous June 16 Bloomsday celebration at the Rosenbach Museum?

"Stately, plump Buck Mulligan," anyone?

Contact architecture critic Inga Saffron at 215-854-2213 or isaffron@phillynews.com.