The colored powder, made of food-grade cornstarch, was safe enough even for children. Several parents ran through the course with strollers, coming through the finish line in matching father-daughter, mother-son tie dye.
The event - billed as the Happiest 5K on the Planet - takes place in 30 countries. Last year Philadelphia hosted the run on the Parkway, with the colorful stage set up in front of the Art Museum.
Both of Sunday's runs - in Philadelphia and Minneapolis and St. Paul, Minn. - were sold out.
Kaitlyn McSurdy, 20, of Ringtown, Pa., ran last year's Color Run and brought a few friends back this year.
"I feel like Barney. I was Baby Bop for a while there, till the end," McSurdy said, invoking the purple and pink dinosaurs from the 1990s children's show Barney and Friends.
Organizers say the Color Run aims to promote fun, health, and happiness and was inspired by similar events such as paint parties, mud runs, and the Indian festival Holi.
Participants and spectators arrived at Fairmount Park before 7 a.m. Sunday, dressed in tutus, wigs, neon sweatbands, and whatever else they might have had left over from Halloween or a 1980s-theme party.
The first runner finished in . . . 12 minutes? Maybe 13 or 14? No one cares, since the Color Run eschews timers.
Nikki O'Neil, 31, of Souderton, said she likes this run better than others she had done, because it is "more lighthearted."
"Mud runs are more physical," she said. "This is like dancing the whole way."
Her mother, Susan O'Neil, agreed. "Usually a 5K, I'm just trudging along, getting tired around the 2-mile mark. But this, with the atmosphere and the color stations to look forward to, it felt way easier than a 5K."
And her father, Patrick O'Neil - he did the worm as he passed through the color checkpoints, getting soaked in pigment that muddied into a shade of brown on his belly.
Patrick O'Neil said he felt the happy vibe even before the run started. "The traffic getting in, people were letting each other merge on Montgomery Drive," he said.
But not all the runners came through the finish line heavily pigmented. The bulk of the coloring came after, at the main stage, where a DJ hosted a club-like dance scene and organized "color throws" every 15 minutes.
When the time came, people would run toward the center to join in.
"Five . . . four . . . three . . . two . . . one," and the crowd tosses spires of deep purple, red, pink, blue, green, and yellow into the air. Within seconds, the distinct colors meld into what looks like a gigantic clown's wig. Two more seconds, and it dissolves into a muddy, pinkish cloud.
"Oh my god, that was so cool!" one woman said.
"I got hit in the face, so that made me happy!" said another.
By 8:30, the crowds were thinning out.
Many said they were heading to Center City for breakfast. Spotting them would be easy - they would be wearing a lot of color and smiles.
Contact Jessica Parks at 610-313-8117, firstname.lastname@example.org or follow on Twitter @JS_Parks.