B’sha’ah tovah!
Staining that is light brown (the color of coffee with milk) with no reddish hue may be disregarded and does not render you niddah.
You should continue to take precautions against becoming niddah from further staining by wearing colored underwear or disposable pantyliners and waiting at least a few seconds (ideally 15 seconds) after urinating before wiping.
While we usually recommend abstaining from intercourse until the staining subsides, this is not a strict requirement. You can wipe yourself externally with a tissue (NOT after urinating), and – if the tissue is clear or a clearly permissible color such as light brown – go ahead with intercourse. If the tissue is stained with a problematic color, we advise waiting until the next day and trying again. After intercourse you should wait a few minutes before getting up and cleaning yourselves. Be sure to use colored towels or tissues to wipe yourselves and don’t look for any staining.
Please feel free to get back to us with any further questions.
B’hatzlacha!
This response has been updated to reflect the rulings of our current Rabbinic Supervisor, Rav Kenneth Auman, regarding waiting before wiping.