Mylands paint is gorgeous — that marble-dust, pigment-rich formulation gives a finish few brands touch — but it sits at the premium end of the price scale, so I understand wanting an alternative.
The best like-for-like is Crown Dark Stone at ΔE 2.7 from the original. Straight talk on the numbers: under 1 is imperceptible, under 2.5 is very close, and 2.7 sits just past that line — so stand it next to the real Amber Grey and you'll clock the difference, but live with it on its own and you'd never know. LRV 28.9, so it sits in that same warm, mushroomy mid-depth as Amber Grey. For my money it's the one to reach for if budget is leading the decision.
Dulux Common Dormouse is a whisker behind at ΔE 2.9, LRV 24.8 — fractionally deeper and a touch more grounded. Dulux Heritage or the trade-counter Dulux lines give you a reliable, widely-stocked option that won't break the bank, and the same rules apply: fine on its own, visible side by side.
Third is COAT Darlington at ΔE 3.5, LRV 30.1. It's a little lighter and reads slightly cooler than Amber Grey, so if your room leans north-facing it might tip a shade greyer than you want. COAT is a cracking eco-credentialled brand though, and worth a sample pot if you're torn.
Here's the honest caveat: none of these is a dead ringer, and a close-ish ΔE gets you the colour, not the finish. Mylands' signature is that deep, chalky richness and the way the pigment catches light — the cheaper matches won't quite replicate that lustre, especially in their flat finishes. If the sheen and sheer depth of Mylands matters to you (say, on a feature wall you'll see in low evening light), pay for the original. If it's a big space and you're watching the budget, Dark Stone or Common Dormouse will do you proud.
Whatever you land on, order sample pots of all three and brush them out next to the real Amber Grey on a bit of lining paper. Move it round the room across the day. ΔE figures get you to the shortlist — your own eyes make the final call.