Under the FIFA Laws of the Game it is not true that a cautioned player must leave. The rules of competition may stipulate that they must.
NFHS (US High School) rules do require that a cautioned player leave the field. A substitution is allowed in this circumstance.
Soccer referee
It usually is called a penalty and is showed by a yellow flag.
In most cases play continues. If it was done on intentionally, it depends on the referee but sometimes a yellow maybe even a red card.
It is a warning
A caution, by showing a yellow card, or a send off, by showing a red card.
sometimes a card and sometimes they wll just speak to the player
Yellow as a warning, red to be sent off.
if it denies a goalscoring opputunity a red, if not yellow and final warning
According to the Laws of the Game, this would be sanctioned by cautioning the goalie and the player that switched and showing them both yellow cards.
His job is to call fouls and make sure the game is fair, fun, and played right.
In the end it's the referee and ONLY the referee who makes the call, but if a linesman sees something illegal on the field he can raise his flag to notify the referee that he has seen something and then tell the referee what he saw.
Referees can wear Blue, Red, Green, Gold (yellow), or Black. The color choice depends on the kits being worn by the teams in a specific match because the referee must be easily distinguishable.