The topic of Marshals and Castellans colour schemes came up on the Facebook page this week.
What we do know is that all Marshals and Castellans are drawn from the Sword Brethren, so all would be able to wear red crosses on black (think you typo'd that in your OP?) if they wished. My more recent HQs follow that pattern. (I was taken by the idea, particularly with Castellans, that their appointments can be temporary, so they may be absorbed back into the Sword Brethren at some point.)
In addition, the more recently painted GW HQ models wear the standard black on white, with either black or gold trim. (the BT images in the V1 8th Ed Codex have a Marshal with black cross, black trim, white pad, and a castellan with black cross, gold trim, white pad.) So I think you can happily mix in some gold to set them apart.
I find it a bit weird there's no official picture of the classic terminator captain in Black Templar colours, he seems a perfect candidate to have been Templar'd as he had a full cloth tabard.
That was indeed a typo, thanks.
Yeah, at first I thought that marshals/command units with the standard black-on-white was a new thing, but then I checked out the old 4th Ed. codex and there's a picture of a Marshal in there that features black-on-white insignia and black trim.
Like you, I plan to go for standard SB colors on Castellans because of the temporary nature of the position.
Edited by Mmmmm Napalm, 18 May 2020 - 03:45 PM.