Can You Get Shiny Koraidon or Miraidon in ‘Pokémon Scarlet’ and ‘Violet’?

Unfortunately, you won’t be unable to obtain Shiny versions of either Koraidon or Miraidon in Scarlet and Violet, as the Legendary Pokémon are Shiny-locked. Though many of the previous Pokémon games have given players the opportunity to capture Shiny versions of the various Legendary Pokémon in the games, these ones are not available.

Since you encounter Koraidon and Miraidon so early on in your gameplay experience, there is no way to soft reset the game and receive a Shiny instead.