We know Númenóreans could have golden hair, like Aldarion.
I'm inclined to think he had golden hair or liked to dress in gold. There's probably no definite answer in canon, but can there be other explanations?
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According to the Tolkien Gateway,
Had Ar-Pharazôn taken a Quenya name, he would have ruled as Tar-Calion. This title does not appear to be a direct translation of his Adûnaic name, since it means "Son of Light" (from cala 'light', and -ion, the masculine patronymic). Like all the other Kings of Númenor who took their royal names in Adûnaic, Pharazôn added the prefix ar ('high', 'King') to his name when he seized the Sceptre.In the Etymologies, Tolkien noted the name Tar-kulu (in the entry for root KUL), which editors Carl F. Hostetter and Patrick H. Wynne suggest could be a formerly unattested name of Ar-Pharazôn.
Also according to the Tolkien Gateway, "Kul" means "golden-red", "flame-colored" or "orange", which suggests a reference to the color of Ar-Pharazon's hair, skin or chosen apparel.
However, without any further notes from Tolkien about the name, we can't know for sure if this is the case. The "golden" could also refer to "favored" or "skilled", as in "golden boy", or to his wealth.