About every season you have new colours. When it's the AO, new colours. Slow HC tournaments afterwhile? New colours. Roland-Garros and prep clay tournaments? New colour. Wimbledon? New colour. US Open series? New colour(s). US Open? New colour. yes, one colour for the USO series and one for the real deal, if not two (day and night session). End year tournaments? New colour. Adidas does the same. So does Asics, Lotto, ect. Maybe with less different colours, but they still do. It also includes clothes, which are changed every season. Think of the shoes accompanying the clothes for a perfectly matched outfit.
In fact, Babolat is pretty unique in the fact they rarely put out new colours for their shoes... Now that Roddick isn't around anymore, it won't help. They still had some Wimbledon custom shoes, but that's because they have a deal with the tournament starting this year.