Is the dirt that appears on underwear after wearing it for a day or more, and is the color of the clothes when they are slightly soiled with dust, considered a sign of impurity?

If one is not certain that the trace found on the garment originated from the private parts, then it is not considered a kudra (brownish discharge). The default is that nothing has exited the private parts until absolute certainty is attained regarding it. This question may stem from the incessant whispering (waswasah) from which people have been warned repeatedly, as it leads to great evil.