My approach would be to take what I assume the properties of felt are in terms of how they relate to a tennis game and assume that those properties are selected for. So, felt is probably a little cushion, giving the ball a little more durability as it gets whacked by rackets or off the ground. The felt provides grip, without which, a tennis ball may bounce off the ground in ways that are impossible to return. The color of the felt makes the ball easier to see (though of course you could color a nude call too).
Something like that.