What color primer did Germany use before painting the Stahlhelm during WW2?

I'm trying to paint a scale model of a WW2 German soldier and want to give the helmet a weathered look by chipping the helmet so it shows the primer underneath the top layer of paint.

Google tells me that German vehicles were primed using Red Oxide primer. Was this also the case with Stahlhelms? If not, are there any records of the color of the primer they used?