The default — and the safest — is trim lighter than the walls. It's how most period homes were done and it works because pale woodwork acts like a picture frame: it crisps up the edges of the room, draws the eye to the architecture, and visually lifts the ceiling. Pair a soft wall like Farrow & Ball Cornforth White or Little Greene French Grey with a clean off-white trim like Farrow & Ball Pointing or Wevet and you've got that classic, considered finish that never looks wrong.
But lighter isn't a rule, it's a starting point. Darker trim is a deliberate, grown-up look — and a cracking one when it's done with intent. Paint the skirting, architrave and doors in something like Farrow & Ball Down Pipe or Mole's Breath against pale walls and you ground the room, giving it weight and a bit of drama. It also hides scuffs far better, which is why dark skirting is the sensible choice in hallways and high-traffic spots.
The one thing you must avoid is trim that's almost-but-not-quite the wall colour by accident. A barely-different shade reads as a mistake, not a decision. If you want a quiet, contemporary feel, go all the way and paint trim the *exact same* colour as the walls — wall, skirting and doors in one shade. That seamless approach (often the same colour in eggshell on the woodwork, matt or eggshell on the walls) makes small rooms feel larger and is bang on trend right now.
So, three honest options: lighter for classic, darker for grounded drama, or matched for modern calm. Pick one on purpose.
Practical bit: whatever you choose, use a proper trim finish — eggshell or satinwood — not the same matt emulsion as the walls. The sheen difference does half the work of defining the woodwork even when the colour's identical. Brush a sample patch on the actual skirting and view it in evening light before committing.