If it's a neutral that can be moved between nations, you could go with a simple ship scheme. So, dark grey/black for the underside, a light grey/white for the main buildingybits, light wood for the deck (assuming it's wood), and a coloured line running round the hull and the upper levels, probably red.
Alternatively, if it's quite VSF and odd looking (rather than a Nef), you could go for layers of brass and bronze. Or paint it in the greens and reds common to traction engines.