October Mist is one of Benjamin Moore's most popular soft sage-greys — a quiet, grounded green that sits beautifully in north and east light. It's a lovely colour, but Benjamin Moore carries a premium price tag in the UK, so wanting a cheaper route is fair enough.
The standout alternative is Dulux Green Ivy (LRV 47.1). It comes in at ΔE 1 from October Mist, which is below the threshold of what the human eye can perceive in normal conditions — put them side by side on a wall and you genuinely won't be able to tell which is which. Dulux is widely stocked, available in proper trade-grade emulsions, and a good chunk cheaper per litre. For most people this is the answer, mate.
If you want other options: Crown Sow Good (LRV 51.1) sits at ΔE 3 — close, but it reads a touch lighter and slightly fresher because of the higher LRV. COAT Next Tuesday (LRV 41.3) is ΔE 3.3 and lands a bit deeper and more saturated, so it'll feel more enveloping on the wall. Both are perfectly usable matches, just not as tight as Green Ivy.
The "but what about" worry: paint colour isn't only about the pigment — sheen and finish change how a colour reads. Benjamin Moore's flat finishes are famously chalky and low-glare, which flatters greens like this. If you go with Dulux, choose a flat matt or matt emulsion rather than a soft sheen to keep that same soft, mineral quality. October Mist in eggshell vs Dulux in vinyl matt won't behave identically even at ΔE 1.
Practical advice: order a sample pot of Green Ivy, paint two coats on a bit of lining paper (A2 size, not a postage stamp), and move it around the room across the day before you commit. A near-perfect match on paper still needs checking in your own light.