The mistake most people make with an accent wall is treating it as an excuse to go wild — grabbing a bold colour with no relationship to the rest of the room. Don't. The accent wall should feel inevitable, like it was always meant to be there.
Start with the wall, not the colour. Pick the wall your eye lands on when you walk in — behind the bed, behind the sofa, the chimney breast. An accent wall in a thoroughfare or behind a door does nothing. If your room has no natural focal wall, you probably don't need an accent at all.
Then pull the colour from the room. The trick to making it look designed rather than impulsive is to choose a tone that already exists in your furnishings, artwork or rug. A deep green picks up the foliage in your curtains; a warm terracotta echoes a leather chair. This is why accent walls in a vacuum so often look dodgy.
For a deep, grown-up green try Farrow & Ball Card Room Green or the moodier Studio Green. For blue, Hague Blue and Inchyra Blue both have real depth without going flat. If you want warmth and drama behind a bed, Farrow & Ball Mahogany or Farrow & Ball Eating Room Red are cracking. Pair any of these against a soft off-white on the remaining walls — Pointing or Slipper Satin — so the accent reads as the star.
"But should the accent be lighter or darker?" Darker, nearly always. A darker accent recedes and frames whatever sits against it; a lighter one tends to look like you ran out of the main paint.
Practical bit: buy a sample pot and paint a big patch — at least A2 size — directly on the wall in question, not on card propped elsewhere. Live with it for two days through morning and evening light before you commit. Colours shift hugely on a north-facing wall versus a sunny one.