Basically, any TPO built before 1955 was green from the outset, BnT after 1963. Anything built 1955-57 was turned out in silver (roof / ends / bogies included; dark colour is dirt), but repainted green 1958-62, BnT after 1963.
This applies to 4 or 6 wheeled, or bogies equally.
Since TPOs tended not to be a bother to painters too often, the very odd vehicle skipped the green and went from unpainted, through nondescript dirt, to BnT without seeing green.
Jimmy O’Dea’s stuff shows filthy-silver alongside green in many types of passenger stock in the 1955-62 period.