In a letter to the Brooklyn Museum on May 21, 1947, Mark Rothko described a series of painted works on paper--one of which was Omen--that were included in his watercolor exhibition at the Mortimer Brandt Gallery in 1946. The letter primarily concerns a work entitled Vessels of Magic, which was acquired from the Brooklyn Museum at the time. Three of the five works identified by Rothko in his letter are Sacrifice, Vessels of Magic, and Omen.
"It was one of five paintings of the same size and shape which were painted almost on the eve of my watercolor exhibition in April 1946. I have looked upon them as a sort of culmination of a period of concerted painting in this medium, and am happy that it is one of those which you have acquired. It occurs to me that the fate of the other four might be of interest to you. One of them is in the possession of Betty Parsons Gallery. The other three have been acquired by Peggy Guggenheim of the Art of This Century, William Folger of San Francisco and J. Draper in Massachusetts. Sincerely hoping that the picture wears well with you, I remain cordially, Mark Rothko." (A. Rudenstine, Peggy Guggenheim Collection, Venice, New York 1985, p. 691)