I would go with a shade or two lighter than Cashmere. It seems like you want understated elegance in this room, and you’ve chosen a stunning wallpaper which provides a lovely texture and anchors the grey palette.

Thank you Sonia and Rachel! Sonia, those pictures are really helpful. I love the idea of velvet for the curtains, it makes a plain colour a bit more interesting. Thank you! Rachel, yes we wanted something really bold on one wall. It's quite a small area because we have a fairly low picture rail, and the bed will cover some of it. We're having two white floor-to-ceiling fitted wardrobes opposite, either side of a chimney breast, and we'll paint the walls a soft colour to match a shade in the paper. It's just this big bay window I'm not sure about - basically one whole wall is window and curtains (it's a typical master bedroom in a 1930s 3 bed semi). Plain seems boring but pattern will be difficult with the wallpaper...

Hi I think something alone the line in a warmer neutral grey paint colour would work, like Farrow & Ball Purbeck stone no.275, Mole's Breath no. 276, or something more on the red tone, Farrow & Ball's London Clay no.244, or Brinjal no.222. Hope thats helpful. https://www.farrow-ball.com/paint-colours

It is lighter since we extended the room and installed a larger window - it was originally the "box room". However, it doesn't get any sun, hence the reason for the yellow based wallpaper. As lovely as the wallpaper is, I think it would be too busy on all four walls, but thank you for your suggestion.