What is the correct criterion for the sun's yellowing? And does the ablution of a person afflicted with سلس (urinary incontinence) become nullified if he prays Asr after the sun's yellowing? And is his prayer permissible after the yellowing?

The time for the Asr prayer extends until sunset. Catching one unit (rak'ah) before sunset is considered to have caught the prayer. It is disliked to delay Asr until the sun yellows without a valid excuse, because the time of yellowing is when the optional time for the prayer ends.

If you are afflicted with incontinence (sals) and it stops at a time sufficient for purification (tahara) and prayer, then you must wait for that time to pray. Delaying the Asr prayer until the yellowing of the sun due to the excuse of incontinence is permissible, provided that your purification remains valid as long as the prayer time has not exited (sunset) or another nullifier, other than the incontinence, occurs.

The yellowing of the sun is recognized by the appearance of yellowness on the ground and walls and the sun's inclination towards setting. Its timing varies depending on the length of the day.