I don't remember much before the late 70s, but think the BR corporate style had (generally) taken hold by then (no doubt there were exceptions that either didn't get repainted when they should have, or had 'local initiatives') and I only associate the green + white with preserved railways, which would be aiming at SR or BR (S) styles.
Think that plain white is unlikely, as would be relatively hard to keep clean, especially in an age when many people smoked. My best guess is a pale crud sort of colour, probably originally an off-white / cream sort of colour.
Kent Rail website might be worth a look as it has a lot of photos of SR stations, the only one that a google search gave with a waiting room picture was on
the Dartford page, taken 1971, which has darker lower panels, but unfortunately is a black + white photo.