Skimming Stone is one of F&B's most-loved warm greige off-whites — soft, slightly pink-grey, and forgiving in most light. But you don't have to pay the F&B premium to get it.
The closest match by a country mile is Dulux Khaki Mists 5 (LRV 68.4), which lands at ΔE 0.7 from the original — that's well inside the "very close" band, so close most people couldn't tell them apart on a wall. Crown Recipe Book (LRV 65.7) is also ΔE 0.7, equally tight. Both are widely stocked and mixable, and you'll save a meaningful chunk per tin versus Farrow & Ball.
If you'd rather a premium feel without the F&B price, COAT Good Intentions (LRV 67) is ΔE 0.9 — properly close, and COAT's flat matt and eggshell are genuinely lovely paints with good coverage. It's the option I'd point you to if you care about the finish quality as much as the colour.
The "but what about" here is finish. A colour match tells you the hue is right — it doesn't tell you the sheen, depth or how the paint flows. Farrow & Ball's Estate Emulsion has a particular chalky depth; the Dulux and Crown matches in standard matt will read very slightly flatter and more uniform. That's not a fault, just a different character. COAT's matt is the nearest in feel to that F&B chalkiness.
One thing worth saying: Skimming Stone can pull warm-to-pinkish in south-facing rooms and a touch muddy in cold north light — that's the colour, not the brand, so the matches will behave identically. Whichever you choose, always test the actual paint on your wall, on both the brightest and darkest sides of the room, and view it across the day before committing.
My pick: COAT Good Intentions if finish matters, Dulux Khaki Mists 5 if you want it cheap and stocked everywhere.