I think I've heard something similar, just couldn't remember the color. I don't want to show something in primer that should be green, or use the wrong color. I'm digging through my resources, including articles on builds, and have found some definite contradictions........ seems like red oxide is a bit over-used, IMO. I have an earlier tank in about the same state of build as this one started in and I want to show the rear cooling louvre cover slightly open on both. I'm leaning towards the exterion green for this area now, since the louvres themselves would be visible from the outside, through the screens.
Also, the tank I'm building is meant to be a factory re-build, common in the later part of the war, in which usable hulls and turrets are combined to get a servicable vehicle, even though they may not be exactly "matched", as far as production dates and/or places go. I have acouple of pics of vehicles that are identified as such, and since I'm using a variety of kit parts anyway, it was a natural.
Anyway, back from CO, maybe get some bench time in here in a bit, plus gotta scare up another camera......