From what I understand, PDF troopers don't have the same level of training as frontline Guard units, so to compare a trained guardsman to a defence trooper would probably be along the lines of comparing guardsmen to conscripts.
Gamewise it would be a case of just keeping it thematic, and avoiding the use of veterans and stormtroopers. As for identifying colours, I'm pretty sure they still have their own heraldry, banners, uniforms and all the organization that a true Guardsman regiment would. You could pretty much paint them how you want.
Nonsense is our Salvation