It is well known now what a yellow and a red card stand for , and when they can be shown by the referee. Bth are for bad fouls and red is for very bad fouls or following two yellow card showing.
The Green card will be shown for very good sportsmanship on he pitch, like stopping the game when a player is injured badly.
It will all be added in the end of the season.
It was tried in Italy B Division. and will be used later on in this season.