A Color Swapper is a mechanism in LEGO City Undercover and LEGO City Undercover: The Chase Begins that is needed to change the color of your Color Gun for objectives.
In LEGO City Undercover, there are two buildings in LEGO City which have Color Swappers for every color in the game. One is at the Art Gallery in Uptown, and the other one is along the main street in Downtown. They are marked with Color Gun icons on the map.
The Super Color Gun Red Brick in LEGO City Undercover can be used to avoid the need of a Color Swapper.