One thing you haven't mentioned is the size of the room. If the room is on the small side, painting the walls in the same colour up to the ceiling will accentuate the height of the ceiling and bring the walls in. Also, using one paint colour on the fireplace wall and a different one on the other walls will break the room up and possibly narrow the room visually. You'd be better off removing the tiles (they would never have been on the hearth originally, and replace with a slate hearth and choosing whatever colour suits you and the room. The picture rail does break up the wall, but that's a good thing, helping with the room's proportions. The room faces west so you will get late afternoon sun at that time of day. Whether it's a bedroom or a lounge (you didn't say) the grey green that you mentioned will be soothing but will become much greyer at night under artificial light. As mentioned earlier, it's worth buying a sample pot, painting some lining paper and hanging large pieces up in different part of the room. Study this at different times of the day and in artificial light before making your decision.

Hello Your design looks great, Have you thought about a sideboards or console table, gives you some storage space and will break up the wall you can then hang mirrors or photos higher up to cover more of the wall. Using different sizes of items such as lamps or candles will help to change the perception of the size of the wall. This console table and mirror would look great in your room. using the white from the doors and adding more subtle pattern like your rug. https://www.cotterellandco.com/liverpool-dressing-table  Or try https://www.cotterellandco.com/sage-green-honey-pine-wood-console  If you have any question about the products please let us know Kirsty Cottrell & Co

The client was unsure what to do with this plain wall. He considered hanging art work but instead decided to turn the wall into a piece of art! The microcement compliments the kitchen units already fitted and brings the whole room together.

If you must, only one wall. If doing two walls you should do all four walls. Two walls doesn't make sense and just looks like a half-done job. To make it a proper feature rather than a randomly wallpapered wall, it should be the wall behind the bed, so I'd finalise the bed position and then do the appropriate wall. If you do the 3m wall and keep the bed on the 4m wall, it'll look like you wallpapered and then realised the bed was better on the other wall i.e. it will look wrong - unless that wall then becomes a different purpose which makes sense (like aforementioned study for example).