Both Dulux Salisbury Stones 3 (LRV 50.6) and Dulux Dune Grass (LRV 50.9) sit at ΔE 2.7 from Paint & Paper Library Stone V. That's just the other side of the very-close line — under 1 is imperceptible, under 2.5 is very close — so call these close: you'll see a whisper of difference if you hold the two chips side by side in daylight, but on a wall, on its own, nobody's going to clock it.
What's worth knowing is *why* there's a gap at all. Stone V is a warm, soft mid-stone with that gentle Paint & Paper Library depth, and Dulux's nearest pair lean very slightly differently in undertone. Both Dulux options sit just above LRV 50, so they'll behave the same way light-wise — that's a calm, light-but-not-bright mid tone that holds up nicely in north-facing rooms without going cold.
My honest steer: if the finish and exact tone matter — say you're matching into existing PPL work or you love that particular Stone V character — buy the real thing. Paint & Paper Library's Architects' range has a quality of finish that's hard to replicate, and no saving consoles you if the tone niggles every time the light comes round. But make no mistake, the saving is real: Dulux comes in meaningfully cheaper per litre, and where the match earns its keep is on larger jobs, trade-spec ceilings, or rental refurbs where budget and availability win — there, a ΔE of 2.7 is more than good enough.
Whichever way you go, order a sample pot of both Salisbury Stones 3 and Dune Grass, paint two A2 boards, and view them at different times of day before committing. Undertone shifts with light far more than any ΔE figure suggests, so let your own eye make the final call rather than the numbers alone. And if you're brushing it out, keep the sheen level consistent between rooms — mismatched finishes will read as a colour difference even when the colour's spot on.