In the early 1960s Riley became interested in colour theory. When tiny spots of pure colour are placed close together they will blend on the retina of the eye. Riley began experimenting with optical movement in painting, using squares, lines, circles and spirals. She used black and white at first but moved on to introduce colour in the mid-1960s. Her paintings produce feelings of movement and dislocation in the viewer, and even illusions of colour in her black and white works. Her work has been widely imitated, and she is known as one of the founders of 'Op Art'.