According to the Santa Anita Bowling Green Club and the Palo Alto Lawn Bowls Club, lawn bowling bowls are sprayed with a spray chalk to mark those that have come into contact with the jack. The jack is the small white ball used as a target in lawn bowling.
While touchers were traditionally chalked with an X mark, as described on MastersGames.com, the benefit of using a modern spray chalk is that it does not require anyone to come into physical contact with the bowls, which could affect the outcome of a game.
A bowl that has been sprayed as a toucher is not considered to be out of play if it falls into the ditch, which is a shallow trench that surrounds the bowling green. Members of bowling clubs often bring their own spray chalk, but in some clubs, it is a responsibility of a person known as the marker, a non-playing bowler who sprays the touchers and keeps score throughout the game.