A screenshot of the Alola Pokédex in one of the new trailers posted on Pokémon’s Japanese YouTube channel shows two different Pokédexes: Alola Pokédex and the MeleMele Pokédex. With three blank spots below those two, it can be assumed that the Alola Pokédex takes into account all Pokémon in the Alola Region, and that the MeleMele Pokédex is island-specific, much like the Central, Coast, and Mountain Kalos Pokédexes were specific to regions of Kalos in Pokémon X and Y.
This would mean is that the island that the player character starts out on is MeleMele Island, since it’s shown in both Pokédexes that the player owns one of the three Pokémon currently seen. What’s interesting about that is the fact that Melemele means “yellow” in the Hawaiian language, indicating that the player would start out on Yellow Island. With the other three islands left blank in the Pokédex, I’m assuming that we’ll see the other islands named after the first generation Pokémon games, to celebrate Pokémon’s 20th anniversary. This would mean Red Island, Blue Island and Green Island; or, roughly translated to Hawaiian, UlaUla Island, Polu Island and OmaOma Island, respectively. It’s possible that all these names could get twisted during localization, but I believe the homage to the first generation will remain intact.
It’s very important to remember that this is all speculation, but we’re extremely excited to see what Pokémon Sun and moon have in store for us on November 18th!