Hi,
Although its probably true that WWI era vehicles were likely painted by brush, the airbrush was actually inveted in the late 1870s and used by some US railroad equipment manufacturers.
Parkerizing would be used at this time but for smaller things like helmets gun parts etc, a whole tank, No. i think they would have used a lead based primer, red oxide or yellow cream colour cadmium lead paint, lead paint being popular during this time.