10. A stain that is found on an object that cannot become impure is not included in the decree.  For example, if she found a stain on the ground or on the toilet, or if she sits on anything that cannot become impure and finds a stain on it.  Furthermore, a stain found on colored cloth is pure therefore a woman should wear colored garments in order to avoid the problem of staining.
     

 

What's Going On - Aino Mekabel Tumah

In general, all clothing or vessels have the ability to become ritually impure through contact with another ritually impure item.  In general, things that are attached to the ground, such as a tree or a toilet (which is firmly affixed to the floor), and of course the ground itself cannot become impure.  Additionally, something that ordinarily could become impure, such as a piece of cloth, cannot if it is not at least 3 tefachim by 3 tefachim.  Stains on any of these items do not render the item impure and by extension do not render the woman impure.

Paper, however, is an issue.  According to the Chasam Sofer paper that is made from cloth has the potential to contract impurity.  However, we no longer make rag paper (for the most part) and modern paper is made from wood pulp and thus in general cannot become impure.

However, there is also a concept in the laws of purity and impurity called "Tumas Masa", impurity that is imparted as a result of carrying something or of resting on something.  Thus, if a stain is on a piece of wood (which cannot become impure) and thepiece of wood is sitting on a chair (that can become impure), then the chair becomes impure as a result of its contact with the piece of wood.  Others reject this restrictive interpretation of Tumas Masa and hold that it applies only in a case of "masa" of something being moved or carried.  Consequently, if a woman is carrying a piece of paper that has menstrual blood on it, she becomes impure.

Therefore, in order to avoid issues relating to Tumas Masa, women are urged not to look at whatever paper they use to wipe themselves after urinating.  If one doesn't see the stain, one cannot become impure as a result of it.  The knowledge that there is a stain on the paper coupled with the fact that the woman has handled that paper after (presumably) the stain was on it would be enough to render her impure.

The end of this seif is the source for the common advice given to women who are prone to spotting to wear colored undergarments.  With little exception, all poskim agree that spotting on a colored cloth does not make a woman impure, because it can never be ascertained with certainty that the stain is in fact uterine blood.  Therefore, except for during the 7 clean days during which a woman should wear white so she can assure herself that she is not spotting, women are encouraged to be colorful.  The Chasam Sofer is the one view I am aware of who says that undergarments that are designed to adhere close to the body are not included in the decree exempting colored garments.  The Chazon Ish rejects this argument.

Even during the 7 clean days, there is some leniency.  One view is that she can wear colored undergarments on days 2 - 6, but not on day 1 or on day 7.  Unless I'm reading it wrong, the Pischei Tshuvah brings a view that requires white undergarments for the first three days but not thereafter.  As a practical matter, halacha encourages women to use white for all 7 days; if a woman has spotting/staining issues during the 7 day period, she can, under the guidance of her Rav, wear colored garments for most of the 7 day period.