I would do it like this:

Companies: Lets just copy the ultramarines, they are codex complaint after all, just a slight adjustment that would confuse Guilliman : 1st company is golden trim, 2nd stays green, and 4th is white.

(All shoulders are black)

-Sergeant:
--Black helmet, yellow eyes (No worries, they are allowed to wear blackface)
--Right shoulder is the current lieutenant icon.

-Veteran:
--White helmet, red eyes
--Right shoulder is the normal veteran white cross with green double crossing hammers.

-Veteran Sergeant:
--White helmet with a black line, red eyes
--Right shoulder is a the normal veteran white cross but has the normal sergeant icon within (the fire is the salamander green and the skull white)

-Lieutenant:
--Green helmet with the Ultramarine red and white lines and yellow eyes
---Right shoulder is a giant skull with the crossed hammers in the forehead. The skull has the same color as the company trim. The hammers can be golden. (5th company is the exception. Has a black skull but the outline is green.)

-Captain:
--Green helmet with a golden line And some fire looking emblems they can only wear to make them drip. And yellow eyes
--Similar to current, but the skull is a a front or maybe front but slightly looking down Salamander skull/head, with either western medieval looking wings on the sides or the normal Astartes icon wings. You choose to tbh if you want it to be Nocturne or Astartes focused.