Bridget Riley's primary concern throughout her career has been the question of how to approach colour. Working initially with an exclusively black and white palette, her repertoire expanded during the 1960s and 1970s to include primary colours. The breakthrough in her approach and implementation of colour came in 1980, following her visit to Egypt the previous year, and the impression the tomb paintings at Luxor had had upon her. On her return to England she recreated these colours in what she called her 'Egyptian palette'. The Ka series heralded a new direction in her work, using these colours for the first time and consequently bringing about a change in the structure of the work, marking the end of the 'curve' paintings she had been producing previously. Riley felt that her new extended palette required the use of a more straightforward pattern and she returned to the use of simple stripes. This self-restriction is characteristic; when questioned on her technique, Riley cited as a guiding principle Stravinsky's The Poetics of Music: 'The more constraints one imposes, the more one frees oneself of the chains that shackle the spirit' (B. Riley, Dialogues on Art, London, 1995, pages not numbered). Riley's work, however, is more than an experiment with colour and form. The pleasures of sight are an abiding concern because of their potential to express emotion and it was her emotional approach that led her to discover that it is the contrast between the colours that defines their nature.