Man on the Street.(photographer Bill Cunningham)

A few summers ago, on upper Fifth Avenue, Bill Cunningham spied a remarkable creature: a woman, in her seventies, with a corona of blue hair--not the muzzy pastel hue associated with bad dye jobs but the irradiant one of Slurpees and laundry detergent. The woman gave Cunningham an idea. Every day for a month, whenever he saw something cerulean (a batik shawl) or aqua (a Hawaiian-print sarong) or azure (a Japanese parasol) coming down the sidewalk, he snapped a picture of it. One morning, he spotted a worker balancing, on his shoulder, a stuffed blue marlin. "I thought, That's it, kid!" he recently recalled. The following Sunday, "On the Street," the street-fashion column ...