There's no over arching rule. SOme parts were supplied already primered in grey or red oxide. Some weren't. Some entire systems were dipped in red oxide. Unless you get to period photos or speak with AFV restorers, you'll only left to guess.

By the way, I suspect you're asking this because you want to show some primer showing through. Do not fall into the fad of chipping and scratching US made vehicles.

Their top coats were often baked on and not prone to chipping. US paint adhesion was extremely strong. What is more realistic for models is to show areas where dust has been worn off by crew/cargo.