The earliest it could have been was 1955 as that is when Pathe News became colour.
I would have thought it was earlier than 1966/67 though. Early 60s I would have said. I believe the modernisation programme started in 61 or 62, too much of the railway semed to be in use for it to have been 66 or 67. Although I am aware the last steam loco was not retired until November 1967.