In the name of Allah, The Most Gracious, The Most Merciful.
If you pray Asr before the sun becomes yellowish, then that is the time which Asr should be performed. But if you delay it until after the sun has become yellowish then it is not permissible except if there is a necessity.
This is because it has two times: the time of necessity and elective time. Elective time starts when Dhuhr time finishes till the sun becomes yell. the time of necessity, it is from the time the sun becomes yellow till sunset. This is what the sunnuah indicates with reapect to Asr Time.
And Allah knows best.
Dr. Khalid al-Muslih
18 / 09 / 1424 AH