The grey and yellow for 121s appeared with their delivery in 1961/2 and the last example repainted black'n'tan was, I think, very early 1967.
The unpainted "silver" (in reality worn and filthy greyish) was still to be seen on the occasional tin van as late as 1963/4.
Black'n'tan started appearing on a widespread basis from early 1963.
The late green livery appeared in 1955 (with the Park Royals) and green carriages could still be seen 1965/66.
The very last GNR coach to lose its GNR brown livery was brake 3rd no. 114 (now at Whitehead) which was only repainted into black'n'tan in 1967, having skipped the green era entirely. Coaches in GNR brown were still to be seen on the UTA at least as late as 1964.
GNR navy & cream could still be seen on the UTA in 1963, and on CIE about the same time. Beware: the GNR livery is NOT what's on the RPSI Cravens - GNR blue in carriages was darker, almost navy. The RPSI's livery is its own.
If we draw out a chart of the above, to illustrate overlaps, quite some variety is possible. With both steam and diesel, and loads of varied goods traffic, and most stations still handling most traffics, we probably have the single most interesting and varied period in recent railway history.