A while ago, I bought a pair of army surplus arctic rated long johns on ebay. "MOD Extreme Weather" or something. There was an indelible brown/yellow stain somewhere near the groin area. The seller told me it was due to a leak of hydraulic fluid at the storage depot.
They were really warm and cost pennies.
Totally OT...
It is a test for hydration.
If you get dark yellow stains then you are not drinking enough fluid