Wearing white underwear made from synthetic fabric during the clean days is subject to halachic debate. However, if the crotch is a natural fabric such as cotton, or if the underwear is a natural/synthetic blend that is majority natural, it is fully acceptable to wear during the clean days.
Regarding white underwear where the crotch is completely or mostly synthetic, we follow the position that a woman who wears this type of underwear throughout her cycle for comfort may also wear it during the clean days. Otherwise, it is preferable to choose white underwear where the crotch is mostly from a natural fabric like cotton.
If a woman finds a stain on a white garment made from synthetic fabric, she should ask a halachic question.
Please feel free to get back to us with any further questions.