What color card does a referee show for serious foul play?

A referee shows a red card for serious foul play as it indicates a severe infraction that warrants immediate expulsion from the game. Serious foul play typically involves actions that not only violate the Laws of the Game but also endanger the safety of another player, such as reckless challenges or excessive force.

The red card serves as a strong disciplinary measure, signaling that the involved player has committed a serious offense, such as a violent tackle or an intentional act that could injure an opponent. By issuing a red card, the referee effectively protects the players and upholds the integrity of the game.

In contrast, the other colors of cards serve different purposes; for example, a yellow card is shown for cautionable offenses, while blue and green cards are not standard cards used in the Laws of the Game.