Rock Salt is one of Dulux's soft off-whites — at an LRV of 82 it's up in near-white territory, so the useful question isn't 'what goes with this colour' but 'what does this white sit over'. It's the top layer of the scheme; the character comes from what you put underneath.
The easiest structure is a three-step ladder. Rock Salt on ceilings, trim and the biggest walls. Then Paint & Paper Library Slate IV (LRV 67.5) — a warm grey-green with an olive lean — one step down for panelling, a companion wall, or the woodwork if you fancy trim deeper than walls. Then Mylands Artichoke BH.13 (LRV 27.6), a muted, earthy olive, as the mid-depth note on joinery or a cupboard run. Three steps, each clearly deeper than the last, and the whole thing reads calm and deliberate.
For the anchor, Dulux Sapphire Springs 1 (LRV 6.4) is a deep, inky slate-blue that gives the scheme a proper full stop — a feature door, a vanity, the back of shelving. Against all that softness, one very dark moment does a lot of work.
The 'but what about warmth?' question: because Slate IV and Artichoke both lean warm and natural, this scheme takes warm materials happily — oak, terracotta, linen, aged brass. Rock Salt itself won't argue; off-whites this light mostly take their cue from the room around them.
Practical advice: paint two coats on big A3+ boards and check them morning and evening, including in the darkest corner of the room. Next to a brilliant white, Rock Salt shows its softness; next to Slate IV it reads almost pure white. Judge it against its actual neighbours, not in isolation.