Mylands paint is gorgeous — that deep, oil-rich pigment load gives a finish few brands touch — but 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. Anything under 2.5 is genuinely imperceptible to the eye, and 2.7 is so close you'd struggle to spot the difference once it's up and dry. 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.
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 colour match is excellent.
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: a tight ΔE gets you the colour, not the finish. Mylands' signature is that buttery depth 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 richness 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.