Is a person considered impure if they find a faint color of feces on their underwear two or three days after having purified themselves thoroughly with water, and is this normal for everyone? And does he remain impure if he uses tissue after water and the situation remains the same? And how can one purify themselves from urine or feces on the body or clothes?

Istinja' (cleaning oneself after defecation/urination) is done by pouring water until the impurity is removed and the area becomes rough. A strong presumption is sufficient. As for the impurity on clothes, it is washed with water until it is removed; merely running water over it is sufficient if the impurity is imperceptible. If you doubt whether something on your body or clothes is impurity or sweat, disregard the doubt. If you are certain it is impurity, then purify yourself and perform ablution (wudu'). If you doubt the time of its excretion, attribute it to the nearest probable time. You might be afflicted with waswasah (obsessive whispering/doubt); therefore, ignore it and do not search to find out if anything has come out of you or not.