Railings is one of Farrow & Ball's best off-blacks — not a flat black at all, but carrying a definite blue-grey undertone. That's the key to pairing it. It works hardest as an architectural anchor: woodwork, ironwork, joinery, a panelled door, or a fully drenched hall where walls, trim and ceiling all go dark.
Because of that cool undertone, you want cool whites alongside it — anything too creamy will look muddy and slightly dirty against the blue. Dulux Moon Shimmer (LRV 92.3) is a clean, bright white that gives you maximum contrast for trim-against-wall or wall-against-Railings-woodwork. If you want something with the faintest whisper of warmth without tipping yellow, Paint & Paper Library Sand I (LRV 95.4) is about as light and crisp as whites get — gorgeous on ceilings above Railings joinery.
For a transitional mid-tone — so you're not jumping straight from near-black to bright white — reach for Mylands Alderman No.60 (LRV 58.8). It's a soft, grounded grey-green that bridges the gap beautifully and stops a Railings scheme feeling cold or graphic.
The "but what about" question: metals and brass. Railings loves polished nickel and chrome — they echo that cool blue undertone perfectly. If you must use brass (and plenty of people do on a dark door), keep it a cool, brushed brass rather than warm antique gold, or it'll warm muddily against the blue and look slightly off.
Practical tip: drench it properly if you're going dark. Half-painted Railings — say, just below a dado — can read heavy. Take it up onto the trim and ceiling and it suddenly feels intentional and architectural rather than gloomy. And always test on the actual wall, north light and all — Railings shifts noticeably between bright sun and dusk.