The original ball used for the sport was not a ball at all. During the first known game, in 1887, players used a boxing glove tied into the shape of a ball. As the game became more refined, the size and color of the ball became more sophisticated and unified. Finally, in 2002, the National Collegiate Athletic Association (NCAA) and the National Association of Intercollegiate Athletics (NAIA) made the familiar bright-yellow hue the official color of their respective leagues' softball.
Softball has different variations--mainly slow-pitch and fast-pitch--and each carries its own set of rules and specifications. The size of the ball, for instance, will vary depending on the rules of each particular league.
According to the official rules of the International Softball Federation, however, a softball has the following general specifications: It is smooth with concealed stitches or a flat surface; may be wound by hand or machine; and has a core made of a rubber/cork mixture or other similar approved materials
The biggest reason for making softballs bright yellow is that they are easier to see. The distance between the pitching mound and the batter's box is approximately 14 feet shorter in softball than in baseball, giving the hitter less time to react to the pitch. The distance between home plate and the other bases also is shorter, giving fielders less time to react to hit balls.
The word "softball" is a bit misleading. As anyone who has ever been hit by a softball will tell you, it is anything but soft. As in baseball, getting hit by the ball hurts--another reason to make it more visible by coloring it bright yellow. The ball is also larger than people who have never played may realize. Ranging from 12 to 16 inches, it can leave quite a large bruise if you are unfortunate enough to be struck by one.
While the 12-inch, yellow ball is the version most often associated with softball, it is by no means the only ball used in recreational or competitive play. Leagues in international competition sometimes use white balls similar to a baseball, and adult recreational leagues are known to use various bright colors like neon-blue or pink. Additionally, the size of the ball and other equipment will vary depending on where you play.