The short version: White cotton you are happy to ruin, sunglasses against the powder, and oil in your hair so the colour washes out.
The dominant aesthetics at Holi in Mathura are get-soaked practical, party, full costume 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: as little fabric as the venue allows, in natural fibres, with a bag small enough to forget about; something identifiable, a colour, a hat, a flag, so your group can find you in a crowd of that size.