It was never clear nor stringent with the color of office for any chapter, but most of the time it's the chapter color only on the shoulder pads (at least the studio minis are painted that way
btw so is the Ravenwing Command Squad Apothecary).
From what you have stated, I think the reason for the completely black armor in the novel is to make them look more unified. The Ravenwing is the topic of the book after all.
Personally I like the paint splatter which the special characters offer to an probably otherwise too boring looking army (generally Astartes armies, not just the Ravenwing), so gladly paint them in their color of office with only the shoulder pads and maybe the backpacks and helmets in chapter color but it's completely up to you