I don't know the exact answer to that question, but I assume that if you encounter it shiny first and knock it out, it won't be guaranteed shiny the next time you see it post-game. In that skarmory video, the person was battling Gonzap in the team Snagem base, which means he didn't snag Skarmory before the final Colosseum. If the skarmory was shiny during that first battle, I'm sure that another video would have been made.
Just a tip about your plans in Colosseum: I'm not sure what exactly you're planning to hunt, but I would try to pick a target accessible after you receive the Master ball. Therefore, you can use the "infinite Master ball" glitch and make your hunt much easier. Just my two cents, but good luck with this incredibly risky endeavor!