If you want October Mist in a Dulux tin, Green Ivy is your match — and it's a cracking one. The measured difference is ΔE 1, which is imperceptible in practice. Stand the two next to each other on a wall and you genuinely won't pick them apart. Green Ivy sits at LRV 47.1, putting it in that mid-tone bracket where October Mist's soft, sage-grey character lives. Same gentle green, same quiet warmth, same way of leaning grey in north light and greener as the sun comes round.
If Green Ivy isn't to hand or your stockist's run dry, Silver Lichen is the backup at ΔE 2.5 (LRV 44.1). That's still classed as very close, but it reads a touch deeper and a shade greyer, so it's not quite the carbon copy Green Ivy is. I'd only go for it if you can't get the first choice.
Now — the honest caveat. A ΔE match gets you the colour, but not the paint. Benjamin Moore's Regal Select and Dulux's emulsions are different formulations with different finishes, different opacity, different sheen at the same nominal level. So the *colour* will match; the way light catches the wall might differ subtly between brands. If you're matching because you're touching up an existing October Mist wall, that's worth knowing — a patch in Green Ivy may sit slightly differently in certain finishes even though the colour's bang on.
Practical advice: order a Dulux Green Ivy sample pot, paint two coats on a bit of lining paper, and prop it against your existing October Mist before committing to a full tin. Soft sage-greys like these are notorious for shifting under artificial light versus daylight, so check it morning, afternoon and evening. If it holds up across all three, you're sorted.