We have a grey sofa, light green chairs, yellow-ey timber floor - what colour walls would be best?

Probably white or another very pale, light colour as that\'s what the furniture is. Maybe grey again but depending on which shade, will make the room gloomy. If it already looks fine with the colour the walls already are, repaint them and if you want the room to have some sort of change, re-arrange the furniture. :) hope I helped your decision making

You could also add a rug or carpet to your floor, to make it less plain.