I tend to favour a slightly darker tone, blue or dark green to avoid blemishes and dirt appearing visible over time and also to reflect a Mediterranean luxury.

Not really Adrian, no. You shouldn't use any other finish such as eggshell, soft sheen, silk etc due to the substrates high porosity so that leaves you with Matt emulsion. There are specific new plaster primers but they are often costly and unnecessary so yet again that leaves you with Matt emulsion. You can water the paint down to 80 percent water, 20 percent paint if you wish or 10 percent water and 90 percent paint - it really doesn't make much difference as long as the substrate is sufficiently primed. The purpose is to prime the new surface so 50/50 is ideal as it blocks out the plaster colour but that doesn't leave you needing to increase your top coat anymore then you need to to block out the white emulsion. All of this is available in any basic decorating book or even google. Don't take it personally but I see you give some really strange advice on here and a lot of it is incorrect which costs people time and money.

We have a room in Setting Plaster, it’s a N facing room and I found that colour looks like a warm neutral in N facing but sickly peach in S facing. Conversely we have a S facing room in Calamine, this looks too sugary pink and almost blue toned in N facing but warms up nicely in S facing. So my advice would be to go less peach and more pink with your choices as you are S facing. As it happens Tani - I have an identical layout to yours, and am also pondering what to paint my white sofa wall! It does look a bit blank and stark in white. I’m going to buy a terracotta/rust/tan sofa but need a colour to go behind it. And some stuff for on the wall. Your plants have inspired me to get some thank you!

https://lornasyson.co.uk/product/bird-fabric-bullfinch-design/ As your bedroom overlooks an orchard you may be interested in Lorna Syson fabric. I have her Great Tit design on one wall in my kitchen/diner but the pink Bullfinch design might look good with Setting Plaster.