It's actually just Ushabti Bone over a white
GW primer, washed with Agrax Earthshade (all over, not recesses only) and edge higlighted with Ushabti bone again. Subsequently I added some faint black lines that are suppossed to be lettering, like you normally do on scrolls, just for some visual interest.
Thanks for the appreciation, I indeed plan to represent personal custodes heraldry by painting each of them differently.
Although I agree that black custodes indeed look badass, shading/highlighting black is a pain in the ***, and I can't seem to make it look acceptable on such large flat surfaces - I painted a Deathwatch Rhino once and it was a disaster, so much I decided to strip it and repaint in red