Holland has not one, but two stunt doubles, and they're both his friends: one specialises in gymnastics and the other in martial arts. In order to ensure you couldn't tell them apart when they're on camera, Flowers "had to make both [stunt doubles'] hair look just like Tom's." Wigs as well as "a lot of [hair] colour" and some expert hair-cutting techniques were all utilised to pull it off.
Speaking of wigs: the stunt doubles aren't the only ones wearing them. Angourie Rice, the actor who plays Betty Brant, got a major bob haircut before filming Spider-Man: Far From Home, which, despite looking amazing on her, wasn't the style the hair department had in mind — so they had to outfit her in a wig as well.