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.

Usually, it'd be the fireplace wall that gets the feature colour. But you also need to consider how that will impact on the visual perception of the space. Also, have you asked your son's opinion? At 11 he is likely to have ideas of what he'd like in his room.

Hi, I would paint the entire wall.

Paint them all Hague Blue. I don’t get the whole painting a feature wall thing - the walls and colour are not the feature, they’re just a backdrop to everything else in the room. Our north facing bedroom is Hague Blue and still feels bright and airy despite the lack of direct sun - white bedding, light flooring and bright accessories