The early tennis ball was "a wad of hair, wool, or cork wrapped in string and cloth or leather." It would have been invented by the people who most likely invented tennis: French monks in the 11th or 12th century. The modern version is a hollow ball, with two different kinds of material covering it: rubber and a fabric. Two similar strips of rubber form the spherical ball and it is then covered by canvas.
Tennis balls are made of a layer of rubber surrounding a hollow centre (core). A thin layer of adhesive covers the rubber and then a layer of felt-like material made up of wool and artificial fibers called the nap. The lettering on the surface is just black ink.
The ball consists of two rubber hemispheres joined together under pressure ( or using a chemical decomposition process to form internal pressure)so that the 'core' formed has a specified resilience and gives the finished ball its required properties. The fabric (felt) is first woven using cotton as a backing (warp)and a wool / nylon for the covering (weft). After felting the fabric is dyed and finished before transportation to a ball making plant. Two 'dogbones' of fabric are cut after application of a latex backing and stuck to the latex covered core - the 'ball' is then cured and the result is a proper tennis ball. Finally logos are applied and the ball packed under pressure to preserve its internal pressure.
Tennis balls used in professional tennis must adhere to ITF and/or USTA rules and regulations (refer to the ITF link, below). One of the rules requires that the ball be either yellow or white, with yellow being the preferred color for better viewing on televisions. For amateur purposes in non-ITF/USTA events, any color is allowed, which is why some retailers sell tennis balls in odd colors, such as pink, green, black, and even multi-tones.