White Cube gallery (London SW1) is now showing Ellsworth Kelly’s single, five-sided canvas entitled ‘Blue Panel’. Alone in its space, Kelly’s masterpiece reaffirms the vitality of American painting of the later 20th century. This painting seems to function almost as a sculptural relief, defining the margin between picture plane and surrounding space. The deep blue exudes a purification of colour within the irregularity of the form. The work is on show through 1 September.