Hi Marcin,
Don't know if you can get it in Poland, but there were two articles on the Crusader AA in the last but one Millitary Modelling Magazine. The first detailled the building of an Italeri Crusader AA 'Skyraker' with wading gear fitted, and the second was a collection of photos and diagrams to accompany the history of the Crusader AA vehicle. I imagine both articles would be a great help to your build, so it might be worth seeing if you could order a copy from their website over the net.
However, to answer your question, I beleive in the article the guy painted his interior white as once complete there is relatively little of the interior exposed compared to the rest of the tank. I imagine due to the nature of the article he had researched this but I could be wrong. It could be possible that the crew of the tank painted the exposed interior OD as they had the same thought as yourself, so you could probably get away with doing either.
Hope this helps,
James