A woman can become a Nidda not only when experiencing an ordinary flow of menstrual blood, but also – under certain circumstances – when she sees a Ketem (stain) on her body or garments. The Sages enacted that even if the woman does not feel any bleeding, a stain which she sees could render her a Nidda and thus forbidden for her husband, depending on several conditions, including the stain’s color and size.
This Halacha is subject to a very important leniency, namely, that a stain can render a woman a Nidda only if it discovered on a white garment. If the stain was seen on a colored garment, we do not consider it menstrual blood, and thus the woman remains Tehora (ritually pure, and thus permissible). For this reason, women are advised upon leaving the Mikveh to always wear colored undergarments. During the seven "clean days" before immersion, a woman is to wear white undergarments, but after immersing, it is advisable for her to wear specifically colored garments. This way, she does not have to be concerned about any stains that are discovered, since they are found on a colored garment.
Hacham Ovadia Yosef, in his Taharat Ha’bayit (vol. 2, p. 388; listen to audio recording for precise citation), cites some Halachic authorities who maintain that this leniency does not apply during the first three days of the seven "clean days." According to this view, we apply a stricter standard during these days, when the woman begins to ascertain the complete cessation of menstrual bleeding, and therefore even stains discovered on colored garments are considered menstrual blood (again, depending on their size and color, and other factors). Therefore, in the view of these Poskim, if a woman, for whatever reason, wears colored undergarments at some point during the first three days, she must have stains checked even if they are seen on colored garments. This is relevant to situations where a woman forgot to wear white undergarments, or in cases where the Rabbis permit wearing colored garments during the seven "clean days" due to certain extenuating circumstances. Hacham Ovadia cites this ruling in the name of several authorities, including the Abneh Paz, the Shaareh De’a, the Nehar Afarsemon, the Zichron Yehuda, and the Pit’heh Teshuba citing the Amudeh Kesef.
However, as Hacham Ovadia proceeds to show, other authorities disagreed with this ruling. These include the Arugat Ha’bosem and the Me’il Sedaka. These Poskim maintain that since the entire Halacha concerning stains was enacted by the Rabbis, and does not apply on the level of Torah law, there is room to be lenient when a stain is found on a colored garment, regardless of when this occurs.
After a lengthy discussion on the topic, Hacham Ovadia concludes (on p. 390) that one may follow the lenient position. Therefore, a stain found on a colored garment may be disregarded, even if this occurs toward the beginning of a woman’s seven "clean days."
Summary: Under certain circumstances, a stain discovered on a woman’s undergarment renders her a Nidda. However, if the stain is discovered on a colored garment, it may be ignored. Even if a woman happens to wear colored garments during the seven "clean days" (when she is supposed to wear white undergarments), she may disregard a stain found on the colored garment.