For the record, a short list of different bugs related to NPC faces in Skyrim, their causes and fixes.

Grey face bug

Symptoms: Face is always of grey color with no make up applied. Changing npc weight with 'setnpcweight' command will temporarily make the face use proper colors. This reverts itself when changing cells and should not be used as a fix.

Cause:

1. There is no facegen (fecegeom i.e. mesh of the head and facetint i.e. texture for head skin color and make up) generated or the path is not right. The mod author either did not generate them or the name of the mod was changed and the path doesn't match the file location. If a plugin is called test.esp and the npc has form id, say 14004085, then the paths for the head data should look like this:
meshes\actors\character\facegendata\facegeom\test.esp\00004085.nif
textures\actors\character\facegendata\facetint\test.esp\00004085.dds

2. A more sinister cause of this bug is when head parts list in the npc record does not match the head parts' names in the nif file. This can happen if the mod author made changes to the head parts list but distributed only the facegen data without the esp file itself. To check this out you need to open the corresponding head nif in nifskope and compare the elements with what is in the npc record viewed in xEdit. When you can't get rid of the grey face bug despite regenerating facegen in Creation Kit then this is usually the case. Dawnguard vampires are notorious for this.

Solution:
Either make sure the proper facegen files are used or - if not - you have to recreate them in CK. To do that click on the actor in CK and press Ctrl+F4. If this didn't work, open the generated nif files and check if head parts match. You can then correct them either changing the names in Nifskope or the matching elements in xEdit/CK.

Brown face bug

Symptoms: Face is plain brown, does not match the body. Nothing you do in game can change that, but starting a new game.

Cause: The facegen data is baked into the save file. This reportedly happens if you used 'setscale' or 'setnpcweight' command on a character. When you then install a mod that changes this npc's looks you get this bug.

Solution: Either start a new game or, if you want to continue on this save, find the npc's baseid (not refid). Open the save file in Fallrim Tools, find the npc by baseid and delete it. This will reset the npcs face but it also resets faction ranks. It is advised to write down the faction ranks showed in Fallrim Tools and then re-add them using console command 'addfac'. Otherwise not everything may work as intended since many mods (like follower frameworks) use faction ranks to identify npcs.

Black face bug

Symptoms: Face becomes extremely dark, almost black. Happens only to some npcs and is not permanent. Saving the game and reloading fixes this. May also affect body.

Cause: Problems with memory for the face/body texture. Either you are using some 8k textures or uncompressed textures for the face/body or the facetint texture of the npc is really big.

Solution: Use Ordenator or similar tool to downsize/compress the texture. You may loose some quality but the bug should stop occuring.

Face and body color mismatch a.k.a. the neck seam

Symptoms: The makeup is properly applied but the color of the body and head differs with a visible seam on the neck.

Cause:

1. The color of the face in the generated facetint is different from the color set in the npc record (QNAM in xEdit).

2. Everything is ok. Texture just blends poorly on the body you use for npc. UNP has some problems with this.

3. Your are using custom bodytypes (i.e. located in a non-standard path) for this race but the npcs have face nifs that still point to standard skyrim path to look for face textures. <-- this is the only case fixed by this patcher

Solution:

Ad 1. Edit the QNAM values to match the skin color in tintmask.

Ad 2. You can use Caliente's Texblend.

Ad 3. Use this patcher. :)