I remember from my hazy memory (I was young then) that in the
past, if it were a NAC bus, it will be white and green for the side
desto. for the feeders that have green/white systems eg 231, the
231W will be completely white. just like what you see
nowadays.
if it is a aircon bus, the top part is dark blue with the words
printed in white. the bottom part color i cannot remember already.

at one point, sbs(t) (cant remember is it before or after the
rebranding to sbst) changed all its plates to shades of red, even
for the buses with the green/white system. they just paint a light
red to signify the green plate.