If you go into spectator mode and click on a mob you can see through the eyes of them, some mobs have different vision than normal. Endermen have inverted colors, Spider see in red and eight eyes, Creepers see in a green static etc. But now all they see is a black screen and the bossbars are white rectangles, normally they are the normal color no matter what mob.

Edited part: Seems it only affects the End dimension. Resource packs do not affect it.