First of all,  this  mod can resolve some of dark face problems, but if it was not helpful to you, ill try to explain why dark face happens. 
Wanna say, this fixes applies to problems with NPC overhauls and its conflicts.

Added some explanation how face discoloration  fix mentioned above works, see in images.


Dark  face ususally caused by 2 reasons:
1)some mode reverting npc appearance plugin changes
2) face  gen data of npc are overwritten

Dark face can occur because of one of these reasons or both. 
So, ill show step by step how to fix any of this reason.

We need SSEdit and keyboad.
As for mod manager, im using mod organizer 2 and dont know how things works in vortex or another mod manager but i try to help if you will have any
 troubles with managers.

To fix reason №1 we need to open you load order in SSEdit and look in your mod that changes appearance .
We need to find our mod and expand it, find Non_player Character(Actor) line and expand it,  
after we need to find Npc with dark face and click on it, click anywhere in right part and click *hide no conflict and empty rows,
 we see that another mod changes NPC, that means we need to make patch. *


You can do one main your *personal* patch for several mods, or, you can do it personally for each mod, your choice. I prefer one main patch.
So we've found our Npc now we need to copy its appearance to patch. Make right click on name of mod in left part of SSEdit and click *copy as override into*
(press *yes i know what im doing* if it asks) and than scroll down to the end of appeared window and click on *newfile.esp* (need to click on check).
You can make patch with esl flag so it wont count in load order, but in mid-game you shoudlnt unistall esl files.
choose *newfile.esp* with ESL inscription in left column.
Name patch how you want and press ok.
Now all appearance change are forwarded but we need to copy all changes made by other mods if you have one.



So we look in left panel and watch if any other mod make changes different from Skyrim.esm (for expmale Ussep make changes to Npc and we need to copy this changes),
but if it changes anything from lines Head Part and lower we do not copy it .

Here i need to copy some records to Patch from a lot of mods, and i drag this records to my patch and drop it. If you have few mods that make same changes you probably
need a patch for this mods and than need to copy from conflicting mods patch to *Our* NPCpatch. If it asks *following files need to be added as masters* click *yes*.



Make sure you know what changes you are coping, for example some mods overwrite Ussep changes and you need to copy changes from this mod not from Ussep, if it needs for proper mod working.
You need to copy everythings: perks, ai data, faction, flags, items, skills, exept appearance and *worn armor* if npc has one. For exmple: If your Npc mod have worn armor, u need to keep it bc probably its a body skin of NPC and NPc will not look normal without it.



So, we copied everything, so click on 3 lines in left corner of SSEdit and click *save*. Press ok. Patch should appear in overwrite folder if you are MO2 user.
Make sure you changed only Our patch, do not make changes to another esp if you dont know what you doing. Place it last in order if you are mid-game,
messing with load order mid game can lead to not good outcomes >_<. Place after Patched mods if you are going to new game.


To fix reason №2 we need to find what mod changed facegen files.

If everything in esp if fine, that means that some mod overwrites FaceGenFiles.

FaceGen files is 2 files named after FormID of NPC, one with file extension .nif, and placed in data/meshes/actors/character/facegendata/facegeom/skyrim.esm/ another is .dds and placed in data/textures/actors/character/facegendata/facetint/skyrim.esm/
For DLC NPC FaceGen Files istead of Skyrim.esm folder name, it will be Heartfire.esm, Dawnguard.esm, and Dragonborn.esm respectively.
FormID is combination of numbers and letters, that can be seen in SSEdit from the left side of NPC name.
For example: Maven FormID will be 0001336A, so her Face gen Files will be ID 0001336A.nif and ID 0001336A.dds



DLC FaceGen Files Names do not completely match with FormID from SSEdit.
FormID of DLC npc in SSedit will starts from 01, 02, 03, but for Facegendata files they name will starts from 00.
When you search for DLC npc, dont forget about it.



To fix FaceGen, we pretend that Maven has dark face, even if we made pacth for her.

If you using Mod Organizer, you able to see what mod are overwrite NPC appearance mod.
Same feature should be in Vortex but im not sure.
So to find this mod we click on our NPC mod, mod that overwrites our shoud highlighted red, mod which being ovewriten by our mod is highlighted green.
In our case, bijin overwrites DibellaBlessed.


Right click on red mod (bijin in our case) and press *information*.
Scroll to conflicts tab,
We see that bijin actually overwrites a lot of files, but we pretend that its not, now we focused on maven.
In right corner we see filter tab, write there mavens FormID 0001336A, now we have 2 files, right click on them and press hide.
Check if there is other mods ovewrites *ours Maven*, and if so, hide those files from them too.
Thats it, Maven should be fixed (or another Npc.)



If for some reason you cant hide Facegens via Modmanager, go in your folder with mods and search via window search FormID of NPC and delete or drop to another folder every file with our FormID name *exept* files in *our* needed mod folder.




If you have plenty of mod appearance you will have a lot of conflicts. And (probably), a lot of dark face bugs.
You need to be sure that NPC from diferent mod match Facegenfiles from this mods.
If you have mod that changes some NPC so you need to be sure that nothing ovewrites FaceGen files of exact this NPC from exact this mod ~*and*~ make sure in esp appearance of exact this NPC is not overwritten.

Its easy to combine different NPC appearance mods, but its very tedious work. You need to load in SSEdit all wished NPC mods, scroll through every NPC in this mods, if few mods changes same NPC, choose one of it, delete all other NPC from other mods or make patch, like we did in step one, but for every NPC with conflicts.
After, provide that all faceGen files match choosen mods files.
Exmple: i want Maven from bijin and Muiri from Dibella Blessed.
I make patch, where i copying appearance of maven from Bijin and Muiri from DB(and providing other changes, made by mods, that not touch appearance).
After, im hiding faceGen of Maven From dibellaBlessed mod Folder and hiding Muiri FaceGen from Bijing mod folder.
So, i have 2 apparently conflicting mods works together :)

If something is not comprehensible, ask away:)

forgot that there is cool tool for NPC apperance mod , if you planning to combine mods, consider using it ^^
 thx to mr Xahid  for reminding me of this tool ^^