Here’s the rule:
GR802 Artificial Markings and Appliances 1. Any change of color or markings other than mane, tail or hoof is prohibited. (Exception: Arabian and Half/Anglo Arabian halter, see AR106; Reining Division; Friesian Division; Paso Fino, see PF101.3). Only clear grooming materials are allowed on the hide and hair. Materials may be used to remove stains.
https://www.usef.org/forms-pubs/34VQYitZ_Zk/gr8-conduct-of-licensed-competitions