Stains that are found on colored underwear do not render you niddah. The fact that a stain is scientifically shown to contain blood does not change its halachic status.

To prevent niddah questions from arising due to this staining, you should continue to wear colored underwear, avoid looking at toilet paper, and take precautions with marital relations. You can review our article on Stains for further details.

Note that bleeding from a wound or lesion outside the uterus is considered dam makkah and does not render you niddah. If your staining is uterine in origin, we would need more information if a niddah question comes up.