Water polo caps are the caps that players wear to identify which team they're on. In most cases, the darker color will be of the home team.
Generally, water polo caps have ear protection. Beyond comfort, the ball or body part smashing onto the ear can rush water into the ear and damage the ear drum.
The numbers on water polo caps and teams begin at 1, with the Goalie Cap, and go upwards matching the total number of players on a team. For example, the number 25 is never put on a water polo cap.