The end sometimes has blue fog reminiscent of the overworld.
This can be seen in a few conditions
1. Entering the end for the first time after starting Minecraft
2. Sometimes when reloading chunks

This however is hard to reproduce. I have discovered pretty consistent reproduction steps via MC-46486

Reproduction Steps

• Set your render distance to 10 chunks
• Enter a bugged state by trying to spectate the ender dragon
• Move away from the island

Observed Results

• The fog will turn overworld blue
  

Expected Results

• This wouldn't happen