A light brown or tan discharge with no hint of a reddish tint does not make a woman niddah, even if found on a bedikah cloth or on a diaphragm. If you still have the bedikah cloth, we suggest that you ask about the brown on it and about your status, since there are some cases in which a woman declaring herself niddah in error is not binding. Since vesatot are calculated based on the onset of bloodflow that has a niddah shade, the halachic assessment of the brown flow you have would also determine whether that forms the basis of your next veset calculations or some subsequent bloodflow does.
For future reference, please note that blood found on a diaphragm can make a woman niddah, as we typically treat a diaphragm similarly to an internal bedikah. Blood found on the hand is treated stringently, especially when the hand has passed directly under the genital area. Declaring oneself niddah generally does have the effect of making one niddah. Therefore, to minimize halachic questions, we typically recommend not looking at the diaphragm before it is washed and/ or washing it in very dim light, we do not recommend performing bedikot when they are not halachically required or feeling for discharge with the hand, and we do recommend consulting with a halachic authority before declaring oneself niddah.
When a woman suspects her menstrual flow has begun, we suggest waiting at least a few seconds (ideally, fifteen seconds) after urinating and then wiping externally with toilet tissue. That way, whatever is found on the toilet tissue will have the status of staining, to which different leniencies apply. (For more details, please see our site’s articles “Ketamim” and “Toilet Paper“.)
Please write back with any follow-up questions.
This response has been updated to reflect the rulings of our current Rabbinic Supervisor, Rav Kenneth Auman, regarding waiting before wiping.