The caps are used to identify both the player and their team, and to protect their ears from injury possibly caused by a water polo ball hitting the head.
The goalkeepers wear quartered red caps, numbered "1", while substitute goalies have caps which are either numbered "1-A" in NCAA games, or "13" in FINA international games.
[1] Thus, each team will have two sets of caps: one white and the other dark colored.
[2] In underwater football, underwater hockey and underwater rugby, water polo caps are worn by competitors to identify which teams they are playing for, and to offer some protection to individuals against the possibility of a burst eardrum caused by the blade of a fin making direct contact across the ear.
[3][4][5][6] Opponents in underwater hockey and underwater rugby wear either one of two colours - white or dark (i.e. blue or black) caps while the water referees wear red caps.