Minecraft wiki entry for Wither, associated text: The wither has a dark purple‌[Java Edition only] or pink‌[Bedrock Edition only] boss health bar that appears only when a player is looking in the wither's direction, including through walls. All withers display a boss bar on the screen. Renamed withers display the changed name above its boss bar. The sky darkens when the wither spawns (a kind of fog).