Well basically this problem is that the color of some vanilla biomes is incorrect especially those of dark tone, this was tested with the following mods:

Cavebiomeapi-1.16.5-1.4.2
Caveandcliffs-1.16.5-7.2.0
Embeddium-0.2.17+Mc1.16.5

Of course in Minecraft 1.16.5 using Forge 1.16.5 - 36.2.41 (All this including the mods with the default settings)

Here are some examples of the Biome Minecraft: Swamp being affected by this problem.

• This is when there is only the mod Embeddium-0.2.17+Mc1.16.5:
  

• And here is when I add the mod Cavesandcliffs Backport and its dependence cavebiomeapi with Embeddium:
  

As you can see the color of the swamp is not correct, this also happens with some modified biomes mods like: oh the biomes you'll go.

The Embeddium team said this about it: This cannot be fixed by Embeddium. Cave Biome API hacks into vanilla's biome system and forces biome retrieval to happen at a fixed Y coordinate. Since Embeddium takes a snapshot of a single 16x16x16 region of the world at a time, in most cases, this will result in biomes being retrieved outside that snapshot, which causes the default biome colors to be used.