Lighter is the conventional move, and for good reason. A ceiling brighter than the walls bounces light back down, makes the room feel taller and the ceiling "disappear" overhead. In a low-ceilinged room, a north-facing room, or anywhere you want airy and open, go lighter. That's the safe bet 80% of the time.
But "lighter" doesn't mean reaching for builder's brilliant white. A stark white ceiling over a soft-coloured wall reads cold and creates a hard line where they meet. The trick is to tint the ceiling — take your wall colour and use a paler relation, or a soft white that shares its undertone. If your walls are Farrow & Ball Cornforth White, a ceiling in Strong White or Wevet sits far more comfortably than anything ice-blue-white. Same logic with Little Greene — pair a wall in a mid tone with Loft White or Slaked Lime above.
Now the contrarian bit. A ceiling darker than the walls, or painted the same colour as them, can be the making of a room. In a tall Victorian room, a deep ceiling in Hague Blue or Studio Green brings the height down and makes the space feel intimate and enveloping — think snugs, dining rooms, bedrooms. And colour-drenching, where walls, ceiling and trim all wear one shade, removes every hard edge and makes a small room feel boundary-less. Mole's Breath or Pigeon done floor-to-ceiling looks properly considered.
The one thing to avoid: a random pure-white ceiling slapped over a thoughtfully chosen wall colour, as if it were an afterthought. It always looks like one.
Practical advice: decide what you want first. Want height and light? Go a shade or two lighter, tinted to match your wall's undertone. Want cosy and dramatic? Match the walls or go darker, and commit fully. Always sample on the actual ceiling — light hits it completely differently up there than on a vertical wall.