GamePress
Once shiny, always shiny.
I came across my first shiny Magikarp today and decided to run some tests to see what someone could do without the karp loosing its shiny form.
You can run away from the encounter and click on it again. You can run away, close and open the game (hard reset). You can run away, move out of the range of the spawn and come back again. You can run away, catch another pokemon and come back. You can run away, spin a pokestop and come back.Did all the five tests above and the karp was still shiny at the end. Basically you can only lose a shiny if it despawns, run away from you, or turns into a Ditto.