From: Rabbi Moshe Sofer
To: Rabbi Naftali
A woman troubled by recurring kesamim cannot rely on wearing a colored undergarment directly on her body, as the Rema's leniency for colored garments applies only to outer garments. The Chasam Sofer rules in accordance with the Rema against Rabbeinu Simcha's stringency, but finds no precedent to extend the leniency to undergarments worn on the skin.
Source: Shu"t Chasam Sofer YD §161