High poly head mod in Skyrim gives my character a dark face

Dark face on a custom head is almost always a mismatch between the head mesh and what the game expects, not a texture problem. Check that you installed everything High Poly Head lists as a requirement, and that nothing loading after it in your list is overwriting the head parts or the face tint. For your own character, opening RaceMenu again and saving once usually forces it to rebuild the face. If it's NPCs going dark, that's the classic face-gen mismatch and the NPC needs its face regenerated for your load order.