Newby Green is, despite the name, a deep slate blue-green that sits in the blues family — teal in warm light, smoky blue in cool — and it comes alive when you stop fighting it with cool, clean whites and instead lean into warmth. The single best thing you can do is drench — walls, woodwork and ceiling all in Newby Green — then bring the contrast through materials: antique brass, smoked oak, old leather. That's what gives it that depth-of-time, lived-in quality rather than a flat painted box.
For accents and pairings, Dulux Copper Glow (LRV 30.1) is the obvious partner. That coppery, terracotta-ish warmth is exactly what a smoky blue-green craves — think a feature alcove, a kitchen island, or just carry it through in metals and textiles. It stops the scheme feeling cold.
When you need a lighter note — trim, a ceiling if you're not going fully tonal, or an adjoining space — reach for a warm off-white, not a bright one. Paint & Paper Library Sand I (LRV 95.4) is properly warm and creamy and sits beautifully against the blue-green without that jarring stripe you get from a blue-white. Farrow & Ball All White (LRV 92) is the cleaner of the two — still soft, but use it only if you want a touch more crispness; I'd steer towards Sand I for most rooms with Newby Green.
For grounding and drama, Mylands Cigar BH.20 (LRV 11.8) is a gorgeous mid tobacco brown — brilliant on a fireplace surround, a chunky skirting, or joinery if you want a tonal-but-darker anchor. It echoes the leather-and-oak palette the colour is begging for.
The one rule: avoid bright white trim. A clean modern white will make Newby Green look slightly dirty and dated rather than rich and intentional. Go tonal (paint the woodwork in Newby Green too) or use a warm off-white. Test your chosen partners on a large board, viewed in the actual room's light, before you commit — Newby Green shifts more than most: teal under warm lamplight, smoky blue in cool daylight.