Ammonite is one of Farrow & Ball's most-loved greys — a soft, warm neutral that sits beautifully in north and south light without going cold. Good news: you don't have to pay the F&B premium to get it.
The pick of the bunch is Dulux Gentle Moon (LRV 66.1), which lands at ΔE 1.3 from Ammonite. Anything under 1 is genuinely imperceptible, and under 2.5 is very close — so at 1.3 you'd struggle to tell them apart on a wall. It's the match I'd reach for first, and Dulux Heritage's emulsion gives you a flatter, more F&B-like finish if you want to close the gap on sheen too.
Next is Crown Sugar Bowl (LRV 69.9) at ΔE 1.5 — also excellent, just a touch lighter and brighter thanks to the higher LRV. If your room is on the darker side, that extra bounce can actually be a bonus.
If you fancy something with a bit more eco credentials and a lovely chalky finish, COAT Rathbone Place (LRV 69.5) comes in at ΔE 1.8 — still very close, water-based, low-VOC, and you can order proper peel-and-stick samples to test before you commit.
Now the honest caveat: a colour match gets you the colour, not the F&B finish. Farrow & Ball's Estate Emulsion has a particular dead-flat, chalky depth that the cheaper trade paints don't quite replicate. For most walls that difference is academic — but in a feature space lit by raking light, the texture can read slightly differently. That's a finish thing, not a colour thing.
My advice: buy a tester of Gentle Moon and Sugar Bowl, paint two coats on lining paper, and move it round the room across a full day. Grey shifts hard with light, and Ammonite has enough warmth in it that you want to see how the match behaves morning and evening before you commit. Get that right and you'll have Ammonite on the wall for a fraction of the cost.