This is a similar home. Does the architect's response help you? "Yes the soffit is painted ( Resene Bokara ) to make it similar to the cladding. We did it this way in order to emphasis the sculptural form of the house - if it was white it would have broken up the form somewhat. I think it depends on what you want to achieve, if the line of the joinery is at ceiling level then it is good to have the ceiling and the soffit the same colour in terms of how you experience the interior space. If the exterior look is more important then the dark soffit would probably work better."

It's great to see professionals consider sustainable options

I like the look of horizontal long run iron would this be a good choice for a house which is exposed to salt bearing southerly wind. Would it be more expensive than traditional wood?

I would look at colour samples next to actual sample of the schist to be used. Take that to a local paint/render shop and look at them together. Looking at your image, the schist looks to be a golden schist rather than a grey. Is this just the way it has been shown? I would check with the suppliers for the actual colour of stone to be used as there are a lot of possibilities and the colour will affect your colour choice for the plaster. Here are a few websites showing some schist colours: https://www.worldofstone.co.nz/stone-mason/stone-masonry/schist https://wanakastone.co.nz/style/ Also consider the visibility of your mortar and its colour. Keep in mind that depending on your render system, you may be even more restricted in LRV, as some systems require an LRV of no less than 40%.