50 Marines? Friday? Thats insane!
On a more serious note, you could simply paint them as fallen (black armour
iirc)
If that's too simple or not fluffy enough for you, there is always file and green stuff...
Question for
DA lore buffs: Do the Fallen use the same iconography as the loyalist
DA?
Edit: checked on fallen and their scheme, they are basicaly black
DA who (sometimes) fight for chaos. You propably still need icons, at least those undivided ones... They are just chaos stars on a stick, but i would reccomend mounting your icons on backpacks (easy to realize and nice for swapping them later on), just make sure they stand out...