Suppose the Green Paint Has Flaked Off a Soviet WW2 Tank...

If paint is worn off, it usually won't show up red oxide, but an orange if it's rusted or dark steel and kind of graphite if the steel is constantly rubbed. I'd guess you're seeing primer. Is the JS a I, II or III? That may also have some bearing.
Here's what I'd do: Figure if the Soviets had time to paint it, they may have had time to prime it. However, I'm sure there were times when no primer was applied and we know for a fact that some left the factories with no paint what so ever. So, it seems like the field is wide open here and I'd go for the most interesting, worn, weathered appearance! (would the 'salt method' work?)
I've seen M60's that I know were primed, not showing any primer when the paint either flaked or was worn down to the bare metal around hatches and stuff.

Finally (whew!) The soviets could possibly have used more than one type of primer (iron and / or zinc oxide... depending on the composition of the metal).

Ron.

This is bugging me now... I have colour pics of a Stalin tank that shows a fair amount of a red oxyde color with the green... Is it just rust? The red oxyde tend to be found on raised areas suggestig to me that the grren paint was eventually scratched over to reveal the red oxyde primer...

Having said that, I also have a pic showing the same vehicle with areas, mostly around the engine grills, showing up in a light yellow/green reminescent of the paint used inside US aircraft...

The inside of the tanks were white(ish). The inside face of the crew's hatches were green however.

You know, Monster, this is a very good question..! I had to think about that one and I'm not certain I can offer you a definitive answer at all. However, I know that there has been reports and pictures of vehicles rushed to battle in an un-painted state, which makes me think that maybe they did not use primer at all, but just the dark olive green...

The Russian white-wash was water-based and normally scrubbed off in the spring, by the way.