With the Lightchecker you can see where mobs can and cannot spawn.
(minecraft.gamepedia.com/Light)

Light Level:
7 - 0 red to black
10 - 8 yellow to orange
14 - 11 white - blue - green

Note:
- This requires OptiFine to work.
- This ignores sunlight level.
- Works with all resolutions.
- Minecraft version 1.9.x - 1.14.x

Lightchecker by MonaLisa is licensed under a Creative Commons Attribution-Share Alike 4.0 International License.
(No Redistribution. Link back to this thread. No money making links. Give credit. Using Lightchecker in screenshots and videos, including monetized, is allowed.)

How to install this resource pack?
- Press Start and type in %appdata%
- Open .minecraft
- Find and open your .minecraft folder
- Drag and drop or copy and paste the .zip file into the resource pack folder
- Boot Minecraft with OptiFine
- Go to options - > resource packs
- Select Lightchecker. Lightchecker needs to be placed on the first position in the list of activated resource packs.
- Done!

Best results with Monsterley

Older Version you can download at Curse.

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Many thanks to TheRealLEGOEPIC for the Update to Minecraft 1.14.x
Lightchecker now shows correct colours and ignores sunlight in all dimensions.