Is it permissible to wear underwear that is not white? Must one change underwear when faint yellow or brown stains appear? What is the ruling on prayer and ablution in this situation? Is it obligatory to make up previous prayers if those stains were impure and invalidated purification?

It is permissible to wear any inner or outer garments unless they involve imitation of women, extravagance, or silk. As for any yellow stains seen on clothing near the front private part or brown stains near the back private part, if one is not certain that they are impurities, then it is not necessary to change the clothes or wash them. Prioritizing the original state, which is purity, means that there is no need to repeat past prayers.