During the 1970s, Hughes began using colour, though in a different way from his partner, Jean Spencer. For her, colour became the dominant means of expression: for Hughes, colour complemented his still complex geometry. This Double Unit work also illustrates the frequently seen feature of his work (as in the maquette) of a juxtaposition of imagery. In this case, the geometric forms which in the unit on the left are in colour, are repeated on the right in greyscale. Thus two images which are identical in size and shape project very different qualities and the eye can move from one to another in a type of dialogue which fascinated Hughes. This work is very similar to one illustrated in the catalogue for the Arts Council's Constructive Context exhibition of 1978.