What should a referee show to identify a player that is sent off before the kick off?

When a player is sent off before the kickoff, the correct procedure for a referee is to show a red card. A red card indicates that the player has committed a serious offense, resulting in their expulsion from the match. This action is crucial as it communicates to everyone present that the player cannot participate in the game.

In most sporting contexts, the yellow card serves as a warning for lesser infractions, while the red card signifies a more severe breach of rules that warrants immediate dismissal. The blue card is not standard in soccer; it is used in some other sports for specific violations. Likewise, a white card does not have a recognized meaning within the framework of soccer officiating.

Thus, using a red card before kick-off appropriately indicates the seriousness of the situation and aligns with the procedures established in the Laws of the Game.