Textile dyes with high resistance to UV light induced fading on poly (ethylene terephthalate) (PET) fibers are important for textile substrates used in applications such as automobile interiors and outdoor furniture. Dye photostability influenced the development of dye structures for automobile fabrics dating back to the 1980s but the nature of currently used disperse dyes is less widely known. As part of a study designed to provide updates on this subject, the current investigation employed experimental and theoretical methods to determine the nature and properties of commercial dyes used to produce lightfast colors on PET fibers. Having completed the characterization of scarlet and blue disperse dyes for PET-based fabrics used outdoors, our attention turned to the characterization of commercially available lightfast yellow dyes. HPLC analysis showed that a pair of yellow products were mixtures containing three or four dyes, and the use of 1NMR, HRMS and single crystal X-ray diffraction analyses indicated that they were homologous azo–anthraquinone based dyes with or without a 1,8-bis(phenylthio)anthraquinone dye. Results from calculating Frontier HOMO and LUMO isosurfaces indicated strong HOMO/LUMO overlap, and the E0-0 for the homologous anthraquinone-based azo dyes was exactly the same (2.37 eV) which contributes to their photostability.