Minecraft wiki entry for Evoker, associated text: The sheep color conversion spell is a reference to the priest units from Age of Empires, where they can turn enemy units into friendly units (changing the unit's color in the process) after making a "wololo" sound.
Because Microsoft controls the copyrights for both Age of Empires and Minecraft, Mojang was able to get the actual "wololo" sound effect to use in the conversion sound effect.
Evokers are referred to as 'evocation illagers' in the code for pre-1.13 and Bedrock Edition code.
Due to a bug, hitting an evoker in survival mode and converting the gamemode into creative mode still causes the evoker to summon fangs and vexes at the player, although they deal no damage.