By the first Sunday in October, the leaves in the Bois de Boulogne, the large, heavily forested park in Paris’s tony 16th arrondissement, begin to turn a beautiful shade of gold. It is on that day that the nearly century-old Prix de l’Arc de Triomphe is held at the Longchamp racecourse along the western edge of the park bordering the Seine. Considered Europe’s most prestigious horse race, its timing influenced the look of the track’s new grandstand, designed by Parisbased Dominique Perrault Architecture (DPA). “It’s a soft color,” says Perrault of the gilded hue of the structure, inaugurated with its first race on April 29. “It’s all about the relationship to the landscape.”