Sadly Frank Lloyd Wright passed away six months prior to the building's opening the next year, at which point artwork was indeed hanging from the 'dead-white' walls.
Transcript follows.

Transcript
Dear Aline!
S.O.S.! Tell "your architect" that the honor of his profession is at stake. A picture-hanger (named Sweeney) is authorized by the Trustees - Harry Guggenheim, Chairman, to barge in on the architect of the museum and paint the interior dead-white - thus tearing the inside from the outer walls of the organic building. Of course they know not what they do. Sweeney is as he has been from the first -- malicious and willing to do anything to destroy the building to set up his own authority. He has no sense of architecture, so no regard for the architect of this one.
Harry (Guggenheim) is puzzled; does not know what Sweeney wants to do to the building and ought to be told by some authority how serious the sabotage would be. A pity to have a masterpiece (it is) daubed to death at the end to gratify a metempsychosis for the white-sepulchre for a museum.
Please get into your fighting clothes and see if some impression can't be made on the Solomon R. Guggenheim Trustees - Harry especially - to let the building alone to come to its desired and destined conclusion with support instead of interference.
You know how - now love me enough to --- help!
Affection!
(Signed, 'Frank Lloyd Wright')
Frank Lloyd Wright
May 24th, 1958
Enclosed a description of the museum sent to l'Architecture d'Aujourdhui, Paris. Also for the reissue of An Autobiography - "Concerning "white-wash".