Hey anon!  Good question.  I imagine both Melkor and Thuringwethil having pretty similar hair–long, straight, and black.  The difference is, Thuringwethil always looks like she just came from the salon, and Melkor looks like he has a severe allergy to hair brushes.  The two of them and Gothmog all have dark brown eyes.  Gothmog’s hair is actually blond, which few people realize because he keeps his head basically shaved.  As usual, Mairon’s the special snowflake of the group.  He’s got red hair that goes a little blonde if he spends enough time in the sun (which he almost never does).  He’s the only one of the group that doesn’t have brown eyes.  His are amber–which, believe it or not, is a real thing that exists.

(If you’re interested, here’s a page with face claims I made a hundred thousand years ago)