If you watch English riders in the show ring, you'll see that many of the women have their hair in a hair net. It usually matches the color of their hair. Why?
It's partly tradition and partly the fact that hair clips, scrunchies, etc, tend not to fit too well under a helmet or hunt cap. Young girls may wear bows that are set further down in their hair or braid their hair instead. Sometimes the hairnet has a bow on it or is set with some kind of "bling" - but this is more often seen in the US. In the UK, plain hairnets are all but required.