As long as the models themselves are clear as to what they represent, paint scheme matters much less.
So, in this example....
Actual Plaguemarine models painted Red and Brass? No issue from me, as I can clearly see what's what just based on the model.
Generic Chaos Marines painted Red and Brass and used as Nurgleseque/Plague Marines. but being used in an army alongside the traditionally red and brass painted Khorne Berserkers? Yeah not so fair.