Early 1960s, I think. I remember a comment in Roger Bacon's column in Flight mag about it clashing with Purple Airways!
Edited to say that my schoolboy diary for 1959 refers to seeing a Heron 'painted 'a vile fluorescent red overall'!! Date was 6 July, so a bit earlier than my estimate.
Last edited by Atcham Tower; 19th Jan 2021 at 15:26.
Reason: New info