At its Madison Avenue headquarters, Sotheby’s kicked off the May auction season early, smack in the middle of the opening of art fairs all around Manhattan, with a solid if unexciting $433.1 million sale of modern and contemporary art, led by an $85.8 million canvas by Mark Rothko. Major auctions at Christie’s and Phillips will follow next week. The equivalent sale last year tallied just $186.1 million. “The sale was robust, and it felt authentic,” said New York adviser Laura Paulson as she exited the sale room partway through the evening.  Related Articles “It was a good sale,” said adviser Jacob King after unsuccessfully bidding on a Warhol. “Not particularly exciting, but good.” The sale opened with 11 works from the estate of legendary New York dealer Robert Mnuchin , who died last year at 92, and his wife Adriana. The group was estimated to sell for in excess of $130 million.…