You can catch the shiny version of Regigigas in Pokemon GO, but only if you are very fortunate after winning a Regigigas Raid.
The exact odds of any Regigigas you encounter in Pokemon GO being shiny is 5%, or 1 in 20. This is the shiny rate for Legendary Pokemon in Pokemon GO.
The best time to catch a shiny Regigigas is during one of the Pokemon GO Regigigas Raid Hours. This is because the number of Regigigas taking place during both of these events is greatly increased.
Of course, the more Regigigas Raids you win, the more likely you are to find a shiny version. All you need is enough Raid Passes to take part in plenty of Raids!

What Color is Shiny Regigigas in Pokemon GO?
The shiny version of Regigigas shares most of the same colors as the standard version, but it has dark purple bands around its shoulders, wrists, and along its chest.
This variation in color sets the shiny version apart from the standard version of Regigigas which has yellow bands across its body and wrists.
Additionally, you are able to catch the shiny versions of all of the Legendary Regis apart from Regidrago and Regieleki. Find out what they look like below:

There are dozens of shiny Legendary Pokemon to catch in Pokemon GO, including Regigigas. Check out out list of every Legendary Pokemon in Pokemon GO so you can find out which shiny Legendaries are missing from your collection!