Exactly!
Yes, I think it ŵas summer '62, and it was indeed a publicity shot, in which, it will noted that "A6" is displayed in an experimental font which was nor perpetrated, and the white line is also not what became standard.
First applications of Black and Tan in traffic did indeed tend to be more AEC railcard than anything else as they were the ICRs of the day, on most lines taking the most important passenger services. While a handful of vehicles were painted in the new livery in late 1962 to see what they looked like, it would be another year before they were commonplace. Thus, 1963 may be taken as the year when black'n'tan actually became the actual livery in practice. Any layout based in 1963-6 will still have many green coaches as well, though.