From the link, the white pants are part of the "Drillich" work uniform. From searching around, these were intended as work uniforms / overalls. You were intended to wear these (there were both pants and jackets) over your actual uniform, and these would take the abuse.

It seems like the early war patterns were simply undyed. Mid-war versions were apparently dyed darker.

Here's a forum with a bunch of pictures of examples: https://www.militariacollectors.network/forums/topic/4042-th...