Archers often shoot according to a whistle system because it is consistent and works well, even in loud conditions. This whistle system is official for tournaments sanctioned by the World Archery Federation (WA), but it is also very common during practice at many different kinds of archery ranges.
At some tournaments this whistle system may be used with beeps or other auditory signals. There may also be a clock, a lighting system or a series of flags to let you know when to shoot. Because there may be differences for each type of competition, you should ask a judge to be sure what each signal means and how the timing will work. Generally, however, these are the colors you will see in competition: