Conduct while Staining
A stain (ketem) makes a woman niddah only if it meets specific criteria outlined in our article on ketamim. Thus, a woman who is experiencing light spotting can often avoid entering the niddah status by wearing colored underwear and not looking at toilet paper. It is completely legitimate, and halachically recommended, to rely on the leniencies built into the laws of stains.
These leniencies apply to the staining many women experience at various points during the reproductive life cycle (e.g., postpartum, perimenopause, while taking the active pills of hormonal contraceptives). They are not intended to prevent a woman from becoming niddah when she menstruates. A woman who experiences a continuous flow of blood* or feels a hargashah becomes niddah even if she is wearing black underwear or pantyliners and doesn't look at toilet paper.
If a woman has spotting that does not meet the criteria for ketamim (and thus does not make her niddah), we recommend that she avoid actual intercourse until she has been clear of staining for about 24 hours. Refraining in this manner is a voluntary precaution, not a halachic requirement, and does not indicate that she considers herself niddah. Other forms of physical affection are permitted.
Abstaining during staining is prudent because it allows time to evaluate the situation and determine whether the staining will develop into a real flow. Also, stains found immediately after intercourse, or on the husband's body, would be subject to the much stricter rules that apply when a hargashah may have been overlooked.
* Note: There is no clearly defined halachic boundary between "staining" and a "continuous flow." In practice, it is often relatively easy to distinguish. If a woman does not need a pad, or uses a pad only to protect her underwear from stains, the leniencies of ketamim probably apply. If the bleeding is heavy enough that she would feel discomfort without a pad, it is probably considered a flow. If she actually sees blood leaving her body, she is niddah. In doubtful situations, a specific halachic question should be asked.