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.
So, if you want a sliver aluminum case and a black band, you'll have to buy the black band separately and swap it in afterwards. Likewise if you want a space gray aluminum case and a colored band, a space black stainless steal case and a leather loop, or any combination not explicitly included in a collection, you'll have to get the watch and an additional band.
If you go to the Apple Watch Online Store, the option you have in preview now will be the options you have for pre-order on April 10 will be the options you have for purchase on April 24.
So, set your purchasing expectations accordingly, and if you need any extra help deciding what you should get, check out our complete Apple Watch Buyers Guide.