The Apple Online Store previews all of the Apple Watch case and band combinations, grouped into collections.
Despite that, some have hoped, come pre-order time, they'd still be able to to mix-and-match cases and bands to get any combination they liked, without having to buy any additional bands. Sadly not. As of right now, you will only be able to order one of the Apple Watches in the case and band pairs as presented in the collections, and that doesn't seem likely to change.
Full story from the iMore Blog...