Ronald O. Perelman, the financier once ranked among America’s wealthiest men, took the witness stand in a New York State Supreme Court courtroom this week, describing how five of his most prized paintings lost their visual brilliance following a fire at his East Hampton estate in 2018. The case, now in its third week before Justice Joel M. Cohen, pits Perelman against insurance powerhouses including affiliates of Lloyd’s of London, Chubb Ltd., and American International Group Inc.