I don't think Balenciaga.com has sales, but other retail stores like Barneys and Neiman Marcus occasionally mark down stragglers from a past season. Not sure if/when holiday colors will go on sale though.
If they never go on sale, there's always ebay and Bonanzle. They're filled with bags from previous seasons so if you miss a color/style, there's a good chance it'll turn up later! You just have to wait