The infamous dark head glitch even though I don't have any NPC-altering mods as far as I know.

[Kendo 2]

Okay, if the gray head happens at game load then it is not texture swaps and bad files in your folder.  The issue lies elsewhere.  IF you're getting a memory bottle neck it will happen during a normal session of play after 40 minutes to an hour.  Fast travel, go in and out of buildings or dungeons, cast a few spells, fight some stuff, etc.  The more visually demanding things you do, the sooner it will happen, IF that is what's happening.

A prime place to test is the plains west of Whiterun.  Fireball a few deer, go to the giant camp and attack them with magic or arrows, raid Fort Greymoor. When that's done go back to Whiterun.  All of the fires and running water in that cell coupled with what you've already done should enough to tax your game.  By that time the game will have already processed a few swaps and you'll probably see the gray heads appear.

 

When I was having this issue I had my game crammed full of mods.  Fixing the meshes, getting rid of textures over 2048x2048 and cuttimg my mod list (I went from over 100 esps to about 20) fixed the gray heads and mismatched bodies.

[jacb]

Can be enb's FixTintGamma in enblocal.ini... although it worked in reverse for me: every NPC was fine while PC looked like that.

 

PS alt+tabbing also helped... sometimes... until the next cell load. Besides, done enough times it crashes even vanilla, so hardly an option.