Easiest match you'll ever get, this one. Dulux Mysterious Teal (LRV 9.5) lands at ΔE 0 from Dulux Heritage Midnight Teal — that's not "close", that's the same colour to your eye. Anything under ΔE 1 is imperceptible, so you can swap one for the other with full confidence.
Why the perfect match? Dulux Heritage and the standard Dulux range come out of the same colour house, so it's no surprise the deep teals overlap. The difference is in the paint itself, not the colour — Heritage uses a flatter, chalkier velvet finish with a touch more depth and a higher-end feel, while standard Dulux gives you the workhorse Diamond Matt, Easycare and trade options. If you love the Midnight Teal colour but want a more durable, scrubbable finish for a hallway or kids' room, dropping down to Mysterious Teal in Diamond Matt is a genuinely sensible move.
The other contender, Raven Plume (LRV 8.4), sits at ΔE 3.2 — close-ish but you'd notice it side by side. It reads a shade darker and pushes more towards a smoky blue-black rather than true teal. Only reach for it if you specifically want that moodier, less green direction. For a straight swap, ignore it and go Mysterious Teal.
A word on these very dark teals generally: with an LRV under 10, they drink light. In a north-facing room they'll go properly inky and atmospheric — gorgeous, but commit to it. South-facing, the green undertone lifts and you get more of that jewel-box teal in daylight.
Practical bit: order a sample pot of Mysterious Teal and paint two coats on a bit of lining paper, then move it round the room across a day. The colour's a perfect match on paper, but the finish sheen will change how it reads — Heritage velvet versus Diamond Matt won't look identical even at ΔE 0. Pick your finish for the job, not just the colour.