If the one you have is made by Silverfox, the green livery is 100% completely wrong, so ripe for respraying! In CIE green livery, it needs to be a lighter green, and a black roof. These railcars were introduced in the early 1950s, and would have been green from the start, but after late 1962 they started repaiting them black'n'tan. So, up to then, all green - but during the mid 1960s you'd have got a train with some as-yet unrepainted carriages still in green, but others newly painted in black'n'tan, so your plan is of course appropriate.
Painting things one livery on one side and another on the other is a trick used by more than a few; I've considered it for some stuff I have too. It will only work, of course, if the ends and roof of the item concerned is the same in both liveries; in this case you are lucky! In both green and B'n'T liveries, CIE railcars had black roofs and ends.
If you're respraying correct CIE green livery, it's a darker green up to mid 50s, lighter later. In both cases, pale green (rather than white) waistline and "flying snail" logo. (The green that S'Fox uses is more like british Southern Region green or UTA green!).