The reason for discoloration are colored polyenes which are formed during thermal and photo-oxidative degradation of PET. In the link below the mechanism is explained (formation of vinylester, polymerization and elimination to polyene). There are stabilizers like phosphorous acid to avoid discoloration during processing but bleaching afterwards is difficult. An symptomatic "healing" is the use of optical brighteners for recycled PET. I am not sure if chlorine or oxygen bleaching would improve the color.

http://cdn.intechopen.com/pdfs/39405/InTech-Degradation_and_recyclability_of_poly_ethylene_terephthalate_.pdf