For the original Soviet version, I assume Army (the Naval version had a two-tone scheme I believe), then I think the only choice is green or green with white parade striping. The interior was also green because it was open topped and a white interior would have stood out easily. When I did mine the only color variation I went for was a bit of wear and tear on the wood benches that showed the green chipped away to reveal the wood color beneath.

If you haven't completed it yet, one thing you might consider is the long rod with the hand crank that runs along the left side of the interior. My research indicates that it was not present on the P as it relates to the later versions with hinged flaps over the rads. And remember to fill the gap between the lower hull and the vertical facing plates onboard of the wheel arches as they were integrated in reality. I have yet to see a model where the builder dealt with this and to make matters worse, they then highlighted the join line at the painting stage.