Destiny Moreno, who is 17, drove to a Sally Beauty Supply near her Seattle home last September with a newly hatched plan. She peppered a grandmotherly employee with questions about hair bleaches, developers and dyes, and the woman asked matter-of-factly if she were coloring her hair.

“I was like, ‘Oh, I’m dyeing my armpit hair,’ ” Ms. Moreno said. “She stared at me shellshocked for five seconds, and then she started laughing and was laughing throughout the entire interaction.”

She settled on Voodoo Blue by Manic Panic, and the next day posted a video on YouTube in which she wears a tank top and proudly raises her arms to show turquoise-colored armpit hair. The video has had more than 264,000 views. After she stopped shaving her underarms a few months before, Ms. Moreno drew plenty of negative comments, and dyeing her armpits bright colors — she has since also gone with hot pink, purple, green, neon yellow and orange — has been something of an act of defiance.