/en.wikipedia.org/wiki/Penalty card. A red card denotes the ejection of a player or team official and can be issued by the referee. A red card is most commonly shown to players who argue with officials or use abusive language towards them. In some sports, including association football and rugby union, a red card may also be issued for serious foul play such as kicking a opponent while down on the field. In tennis, a red flag serves as a warning that a ball is about to be hit into the audience area, which requires a player or umpire to leave his or her position. If this occurs without notification, the score is changed to match the number of balls remaining in the game. A yellow flag indicates that the ball has been spotted in an unsafe manner (e.g., with the feet). This does not require any action to be taken by the umpire; however, he or she may give a verbal warning if there is reason to do so. A green flag means that the ball is in play and that the player may return it. In basketball, when the ball is visible to the referee, he/she will often blow the whistle to indicate that play has resumed.