Mayim Rabim

Well, I went searching, because comments on this had me curious.

In short, no one had heard of the custom I thought I remembered being taught, that of wearing white underwear on vestos (anticipatory days).

The sefer I looked in was Rabbi Binyomin Forst's "The Laws of Niddah." I found nothing in the many chapters on vestos, so I finally looked under kesems with regards to colored underwear. Interestingly, I found this in the footnotes, on page 208 of volume 1:

27. Rambam writes that Chazal established that a woman should wear colored garments. Kesef Mishnah and Beis Yosef 190 note that the Talmud (Niddah 61b) seems to indicate that a woman may wear, not should wear colored garments. Rema 190:10 seems to follow Rambam's opinion that women should wear colored garments. However, Toras HaShelamim para14 explains that Rema merely advises that a woman wear colored garments to prevent kesamim problems. Chochmas Adam 113:9 and Chasam Sofer, Y.D. 161 seem to follow Toras HaShelamim's approach.

and, more to the point:

31. See Pischei Teshuvah para22, citing Amudei Kesef. Amudei Kesef also writes that a woman should not wear colored garments on her "niddah days." His intent is unclear. Badei HaShulchan (Tziyunim 190:205) interprets this to mean her expected vest day. Thus, a woman is required to don white during the days that she expects her period. Igros Moshe also suggests that a woman wear white during her vest day as a means of effecting a constant bedikah. However, since all other authorities make no mention of this halacha which relates to all women, they seem to disagree.

Which seems to say to me, that while I didn't necessarily make it up, it doesn't seem to be halacha l'maaseh and is certainly not obligatory on anyone who doesn't currently hold by this custom. Oh, and I also submitted a question on this topic to Nishmat... I'll try to post their answer here as well.