The game of bingo has not always used bingo balls. Actually in the beginning, number counters were used instead. These were essentially wooden disks with numbers imprinted on them. They were pulled from either a bag or a box. The number was then called out by the bingo caller.
Eventually these wooden disks evolved into wooden balls. While there isn’t a clear reason why this change occurred there is some speculation. It possible could have been done simply out of the need to make the game more efficient. The balls could be placed in a hopper and spun with one rolling out. Some also wonder if the change occurred to make choosing a number more arbitrary.
Today, modern bingo balls are made of plastic and look much like ping pong balls. American bingo halls play with 75 balls in their bingo hopper while European bingo halls played with 90 balls. Most bingo balls are white with black numbers. However, this can vary and the balls can be made in a multitude of colours.