Bit late joining this thread (as usual), too busy modelling!
The off-white colour for IOM 4 wheelers looks right, it shouldn't be creamy, but the white 'window' band should start just below window level and go right up to roof (i.e. no green band above the windows). The current 'modern' style IOM paint banding started post WW2 with the red and cream livery.
If there are new batches to be made of the 4 wheelers, then a choice of Green/Off-White livery as above (first class) and a Purple-Dark Brown livery (2nd/3rd class) would be perfect, both types with grey roofs and black chassis / running gear. The later Purple-Dark Brown with Off-White upper panels livery (for all classes) lasted somewhat longer, looked lovely but may present Accucraft with a price challenge too far, but not too difficult for modellers to achieve themselves with careful panel masking.
Ed