Vanillin can do that to a brush.
This is from the B&M Website:
The vanillin in Lavanille may cause it to discolor animal hair shaving brushes. Use of a dedicated brush or a synthetic-fiber shaving brush is recommended.
This is from the B&M Website:
The vanillin in Lavanille may cause it to discolor animal hair shaving brushes. Use of a dedicated brush or a synthetic-fiber shaving brush is recommended.