US and British tanks were also painted in a Red Oxide type primer. There is some talk of a grey primer on some Shermans, but I have never been able to have that confirmed. It was standard practice for Heavy machinery at the time, and was carried over to armoured vehicle production.
The Russians initially primed their tanks in red-oxide, but as Stalingrad got a little hectic they by passed that and went straight to paint and finally they sent them out bare metal and often with an unfortunate factory worker as crewman!!!!