Stiffkey Blue is a deep, moody navy with a touch of warmth in it, and the biggest mistake people make is treating it like a feature wall surrounded by cold white. Don't. This colour wants to be drenched — walls, woodwork, and ideally the ceiling too, in varying sheens so the light catches the joinery differently to the flat walls. Drenched, it becomes enveloping and expensive-looking rather than a single sad dark wall.
For accents, go warm and slightly tarnished. Dulux Copper Glow (LRV 30.1) is a cracking partner — a burnished terracotta-bronze that picks up the warmth buried in the blue and stops the whole scheme going cold. For textiles, woodwork details or a contrasting joinery piece, Mylands Cigar BH.20 (LRV 11.8) is gorgeous — a deep tobacco brown that reads almost like leather against the navy.
If you want a touch of light to lift things — a mantelpiece, a ceiling in a smaller room, or trim if you genuinely can't face drenching — reach for a warm off-white. Paint & Paper Library Sand I (LRV 95.4) is properly soft and creamy, and Farrow & Ball All White (LRV 92) is the cleanest of the warm whites without tipping into cold brilliant-white territory. Avoid anything with a blue or grey undertone next to Stiffkey — it'll make the navy look flat and the white look dirty.
The "but what about metals?" question comes up every time. Keep them warm and aged: antique brass, unlacquered bronze, a bit of patina. Chrome and polished nickel fight the warmth and make the room feel clinical.
Practical advice: paint a decent A2 patch and live with it for a couple of days, because Stiffkey shifts hard between daylight and lamplight — it can look near-black in a north-facing room at dusk. If that worries you, it's a sign you should be drenching and leaning into the drama rather than fighting it.