The short version: White head to toe with a red neckerchief and sash. Worn by essentially everyone, all nine days.
The dominant aesthetics at San Fermín are full costume, party and parade. That is the visual language of the site, the stages, the merch, the crowd photos, and outfits that speak it look intentional rather than borrowed.
This is a costume festival, and that changes the maths: turning up in normal clothes is the conspicuous choice. Budget for one full look rather than three half-hearted ones, and make sure you can walk, dance and queue in it for eight hours.
What people actually wear: one good layer over a light base, because the temperature halves after sunset; something identifiable, a colour, a hat, a flag, so your group can find you in a crowd of that size.