Does a change in the color of underwear to yellow at the rear invalidate past prayers, necessitating their reperformance?

It appears that the trace of yellowness at the back of the clothing is from the effect of feces, and therefore it is an impurity that must be removed. If it is highly probable that it is sweat and there is no smell of impurity, then nothing is required, but it is preferable to wash it before prayer. If the impurity is not known until after the prayer, then the prayer does not need to be repeated. However, if it is known and prayer is performed with it before removing it, then the prayer must be repeated.