Paintball Effects is a feature in Call of Duty: Advanced Warfare for multiplayer and Exo Survival. It can be activated/deactivated at any time simply by going in the Options menu and turning it on/off.
When turned on, it does multiple things: blood that comes out of enemies is replaced with paint; instead of bullet holes in walls and objects, splatters of paint is left behind; and the blood on the player's screen indicating when they are heavily damaged is replaced with paint. Killing an enemy while holding the Combat Knife also causes paint to splatter upon the screen. The paint is yellowish-green.