Colors different light levels (0, 7, 11) so you know where to put light sources to prevent mob spawning. Implemented using custom lightmaps.
Custom lightmap support is currently not available in Polytone for the most recent Minecraft versions, so you may only be able to use the resource pack with OptiFine.
When using OptiFine, requires: Custom Colors: ON, Smooth Lighting: OFF, Dynamic Lights: OFF
Remember to check server rules before using resource packs like this in multiplayer.