Jumping back a couple posts about shiny legendaries. In B/W, Zekrom and Reshiram can never show up as shiny unless you do the whole have a full PC, try to catch, force the story to advance and then go back and re-catch them thing, unless that happens they're specifically coded to never be shiny. Tornadus and Thundurus are like Lati@s, everything about them is determined when you start your file, so reloading or re-catching to try and change anything about them doesn't do anything. Any other free standing legendaries are capable of being shiny, and if you catch them as such the game registers the shiny version as the default form in the pokedex.
As for Victini, I won't hesitate to say I'm not 100% sure on that one, he may be coded like Reshi and Zek on Liberty Island, it may be impossible for him to load shiny.
XI Wiki



