Brown is one of the most forgiving walls you can have, mate — it's essentially a deep neutral, so it plays nicely with a huge range of colours. The trick is choosing companions that bring out the undertone you want.
To lift and soften it, go for a warm off-white or barely-there cream on your trim, ceiling and woodwork. Paint & Paper Library Sand I is cracking for this — at LRV 95.4 it's about as light as paint gets, so it bounces light around and stops the brown closing the room in. Use it on skirting, architrave and ceiling to frame the brown rather than fight it.
To ground brown and make it feel considered, reach for a muted, earthy green. Mylands Artichoke BH.13 (LRV 27.6) sits in lovely harmony with brown because they're tonal cousins — both natural, both slightly dusty. Think upholstery, a feature joinery run, or the lower half of a panelled wall. This is the sophisticated, lived-in look you see in good country houses.
To bring drama, a deep blue is the classic high-contrast move. Dulux Sapphire Springs 1 (LRV 6.4) is properly dark and rich — pair it with brown for a moody, layered scheme. Velvet cushions, a painted alcove, or window dressings work brilliantly here. Brown and deep blue is a genuinely smart, masculine combination that never feels gimmicky.
The "but what about" question: don't overload on grey. Cool greys clash with brown's warmth and make the whole thing look muddy and dated. If you want a neutral, keep it warm.
Practical tip — work out whether your brown leans red, yellow or green by holding a plain white card against it in daylight. A red-leaning brown sings with greens; a yellow-leaning brown takes blue beautifully. Always test big sample patches on more than one wall before you commit.