Gay as Paint is a bygone term that originally was used to describe something that had been painted in bright vivid colors (when gay meant bright, vivacious and sparkling before its association with homosexuals). Thus, to be gay as paint meant something was as "gay (bright) as paint...made it possible"
Jacqueline Onassis has been quoted using that term to describe Truman Capote. In a conversation she had with Joan Kennedy, the latter asked if Capote was gay... Jackie replied, "He's so gay, he's gay as paint." By then the term had morphed to describe flamboyant homosexuals in a tongue-in-cheek nod to the reframing of gay as a homosexual term.
"He's so gay, he is gay as paint."