PARIS—It isn't easy turning 40, especially when you're the tallest and—many would say—ugliest building in the capital of love.

Built from steel and tobacco-colored glass, the 59-story Tour Montparnasse rises like a tombstone above Paris's idyllic canopy of low-lying flats. Postcards shun it in favor of its slimmer, more glamorous sister: the Eiffel Tower. The mayor has publicly yearned for its demolition, and a former president was so struck by the skyscraper he ensured it would be the last one ever built in the center of town.

"Where's the nicest place to have lunch in Paris?…The Tour Montparnasse, so you can't see it!'" says urban planner Virginie Picon-Lefebvre, reciting a commonly heard Parisian punch line.

Tired of the derision, the Tour Montparnasse and its defenders this month celebrated the building's 40th birthday with a grand makeover and a rock concert.

Regis Clouzet, a 46-year-old lighting designer, was called in to decorate the building's surface with multicolored lights that flash and flicker to the rhythm of music. The building's owners have plans to replace the high-rise's brown windows with more eye-pleasing panes, but that project has yet to break ground.