It is not permissible to offer any Nafl (voluntary) prayer after Asr! However, this time is not Makrooh (disliked) for Fard (obligatory) prayers. Just as the current Fard prayer of Asr can be offered at this time, similarly, it is also permitted to offer Qada (missed) prayers. However, when the sun turns yellow (pale), then it will not be permissible to offer any prayer other than the current prayer. (Kitab-un-Nawazil / Vol: 3 / Pg: 263)