To celebrate the release of Pokémon Brilliant Diamond and Shining Pearl as well as Pokémon LEGENDS Arceus, from October 22nd, 2021 until November 18th, 2021, video game retailers nationwide in Japan will be giving away alternate-colored Legendary Pokémon for Pokémon Sword and Shield!
If you show the staff at participating stores the HOME menu screen of your Nintendo Switch console with one of the software icons of Pokémon Sword or Shield displayed, you will receive a different color Zamazenta for Pokémon Sword and a different color Zacian for Pokémon Shield.
Zacian has a Rusted Sword item and Zamazenta has a Rusted Shield, so you can change the form of each.
Source:
https://www.pokemon.co.jp/info/2021/09/210928_gm01.html