Jamieson, Fausset and Brown on Isaiah 64:6

6. unclean thing--legally unclean, as a leper. True of Israel, everywhere now cut off by unbelief and by God's judgments from the congregation of the saints.

righteousness--plural, "uncleanness" extended to every particular act of theirs, even to their prayers and praises. True of the best doings of the unregenerate (Php 3:6-8; Tit 1:15; He 11:6).

filthy rags--literally, a "menstruous rag" (Le 15:33; 20:18; La 1:17).

fade ... leaf--(Psa 90:5, 6).