I talked with a guy working at the assembly plant in Los Angeles after he returned from WW2 stationed in Hawaii. He worked there until 1973 when he retired.
All parts arriving to the plant was unloaded and if the parts were needed right away the parts bin were sent to the assembly line right away. That meant that the parts were natural with nothing on. If parts was going to be stored for a longer time, parts was placed on a conveyor and got a shot of black paint just to cover. No perfect paint, only to cover so the parts didn't arrive dark rust red at the line later on.
Some parts were strored outside before ending up on the assembly line. For that reason parts got rusty.
Due to this procedure the parts for a front axle would be natural, black or rust red!!