Shimmer, or iridescence, is an aesthetic feature that was given to the original Kickstarter backers. This allows the original supporters of the game to be recognized apart from the rest of the player base. It can exist on top of any Kickstarter skin, regardless whether or not they were unlockable or natural spawn in the game.
Kickstarter backers can apply shimmer to any Kickstarter skin that they own via a checkbox in the Biology section of their character panel. For example, a Kickstarter must have the White Dragon Skin DLC in order to apply shimmer to a Leucistic dragon.
Once shimmer is applied, it cannot be taken off.