Works such as Deny and 19 Greys conceived in 1966 and 1967 reflect Bridget Riley's developing interest in tone. An interest it seems, which grew from her study of black and white, which had occupied her earliest work. In conversation with Robert Kudielka in 1978, Riley commented on these works saying, 'I wanted something which operated on more levels, was capable of more development, had a more 'grey'd' quality, like the indeterminate nature of reality.' (In Conversation with Robert Kudielka, 1978, see Ed. Paul Moorhouse, Bridget Riley, London, 2003, p.210).