Instagram/@dualipaThere’s only so much you can do with your hair when it’s 90 degrees outside. Leave it down, and you're fighting the humidity. Throw on a baseball cap, and you're committed to hat hair for the rest of the day. Reach for a straw hat, and you're stuck carrying it around the second you step indoors.There is, however, one hair accessory that’s just as practical as it is pretty: the bandana. After a brief Y2K revival, the humble scar…