James Panero reviews six exhibitions currently on view in New York: Thornton Willis: Steps at Elizabeth Harris Gallery (through April 13), Painted on 21st Street: Helen Frankenthaler from 1950 to 1959 at Gagosian Gallery (through April 13), Sanford Wurmfeld: Color Visions 1966–2013 at the Hunter College/Times Square Gallery (through April 20), Judith Braun: May I Draw at Joe Sheftel Gallery (through April 21), Paul D’Agostino: Twilit Ensembles at Pocket Utopia (through April 21), and Joe Zucker: Empire Descending a Staircase at Mary Boone Gallery (through April 27).