There was a short story "the Tyrant's Champion" by Sarah Cawkwell about a Red Corsair who was originally III Legion.
In it he describes his armor has having been painted "a variety of shades from blazing scarlet of the helm to the arterial crimson of the boots"
While it's only one example, it makes sense that a Marine would inevitably take something of his past along with him be it painting his armour in different shades of the warbands colour to Word Bearer carving cuneiform into his shoulder guards or a World Eater hanging skulls off his belt and incorporating god runes into his armour design.
Then again there is the Black Legion Supplement approach which has strikes of colour added to the armour for Slaanesh, Khorne and Tzeentch as well as different levels of ornamentation. The Nurglites are as expected kinda worse for wear.
If I add some Death Guard plague marines to my warband I'd definitely use red for their armour but with plenty of rust, aged gold trim and assorted nasty stuff.
Hope that helps as I'd love to see what you guys (and the voices) do with the Cult troops