A Mark Rothko painting formerly owned by the influential art dealer Robert Mnuchin sold at Sotheby’s on Thursday night for $85.8 million, just barely failing to re-set the Abstract Expressionist’s auction record. Still, the painting is officially the second-most expensive Rothko ever sold at auction. Rothko’s Color Field paintings of the postwar era regularly sell for vast sums at auction . But few have sold for quite as much as Brown and Blacks in Reds , the 1957 abstraction that was auctioned this week as part of a sale devoted to Mnuchin’s collection. It is one of the most expensive artworks to be sold by any auction house this week in New York. Related Articles Ahead of the sale, Sotheby’s said on its website that the painting had both a guarantee and an irrevocable bid, both of which ensured that this work would be sold.…