Squid Ink is one of Paint & Paper Library's brilliant deep charcoals — that almost-black slate with just enough blue-grey depth to stop it reading flat. Lovely colour, but the price tag stings if you're doing a whole room.
Your best-value swap is Dulux Smokey Slate (LRV 12.3), which lands at ΔE 3.6 from the original. That's not an imperceptible match — anything under 2.5 is what I'd call very close — but at 3.6 you're in territory where, on a wall, in normal light, virtually nobody would clock the difference. The slightly lower LRV means it reads a touch deeper, which honestly suits a colour like this. For a dark drama wall, Smokey Slate is the sensible call.
Crown Work of Art (LRV 9.5) sits at ΔE 4.3 — darker again and a bit further from the source. If you actually want a moodier, blacker finish than Squid Ink, that's no bad thing, but as a faithful dupe it's the weaker of the two budget options.
COAT The Coal Drop (LRV 14) is the lightest of the bunch and the loosest match at ΔE 6.1. That's far enough that I'd treat it as its own colour rather than a Squid Ink stand-in — though COAT's eco credentials and low-VOC formula are a genuine draw if that matters to you.
The usual caveat with very dark colours: ΔE figures are measured under controlled light, but deep tones shift hard depending on what's bouncing around the room. A north-facing space will pull all three colder and inkier; warm artificial light softens them.
Do yourself a favour and get sample pots of Smokey Slate and Squid Ink both, paint two A4 boards, and stand them side by side on the actual wall at different times of day. With darks especially, the board test is non-negotiable — and Smokey Slate will almost certainly win you over once it's up.