Chaos Cultists and Corrupted Guard

So with the new cultists out, and having the same weapons and statline as IG guardsmen, I was going to "corrupt" my IG so that they can serve as both Cultists or IG allies with a decidedly Chaos flavour, but how to do it?
Should I use a cutting/burning tool to carve symbols into arms?
Add gasmasks/bandanas to the faces?
Paint on symbols?
Add mutations?

Any suggestions?

All the above! even just some obviously vandalized, or painted over/defaced iconography, combined with seeding the squads with the cultist models will go a long way in itself.

Yeah, I figured for taking cultists I could add some guard models into the cultists squads to bulk them up, and vice versa when using them as allied guard units.
Is painting them enough? I figure they are not overly corrupted looking, more worn down or shabby and could achieve a cohesive look maybe with the right paint scheme?