The type of stains you described are not unusual for women going through menopause. If the stain was found on colored underwear, you are not niddah.
If you found the stain on white underwear you should show the stain to a rabbi in order to determine whether the color was problematic. Brown is often not a problematic color, but a judgment cannot be made without seeing the color.
If you found the stain on toilet paper, your status will depend on several factors, whether you are Ashkenazi or Sephardi, how soon after urination you wiped yourself, and the color of the stain. Please see our article “Toilet Paper” for more details, and get back to us with a specific question if necessary.
Whenever a stain is found, we strongly recommend refraining from relations for 24 hours without considering yourself niddah. If the staining turns into a flow similar to your regular period, you will become niddah. Try if possible to avoid becoming niddah unnecessarily from these stains by wearing colored underwear and not looking at toilet paper. If a stain does not put you in niddah, then there is no requirement to immerse prior to relations.
If your staining is prolonged, you may wish to consult your physician.