If you're after a Dulux stand-in for Paint & Paper Library Mockingbird, go for Dulux Mysterious Teal. It comes in at ΔE 1.4 from the original, which is genuinely close — anything under 2.5 is a very tight match, and 1.4 is the sort of difference you'd struggle to spot once it's on the wall and the light's playing across it. Its LRV of 9.5 also keeps it in the same deep, inky register as Mockingbird, so you won't lose that brooding teal character.
The other contender, Dulux Indigo Shade (LRV 8), lands at ΔE 3.9. That's a noticeable step away — it pulls cooler and bluer, so it reads as a different colour in the same family rather than a true match. I'd only reach for it if Mysterious Teal isn't available or you actually fancy nudging things a touch more blue-grey.
Now, the honest bit: matching across brands is never perfect, because the base paint and pigment recipe differ. A measured colour match gets you the hue, but Paint & Paper Library's paints have a particular depth and chalky-soft finish that Dulux won't replicate exactly. If it's the *look* of Mockingbird you fell for — not just the colour code — you may notice the Dulux version sits slightly flatter, especially in their matt emulsion.
My advice: order a Dulux Mysterious Teal sample pot and a Mockingbird tester, paint both on the same offcut of lining paper, and view them side by side in the actual room across a full day. Deep teals shift dramatically between daylight and lamplight, and a north-facing room will read these very differently to a south-facing one. If the match holds up by eye, you've got yourself a cracking budget-friendly alternative. If you can already see the difference and it's the original you really want, stick with Mockingbird — it's worth the extra for a colour this striking.