For warm paint colours, 2700K is your default. It's the warm white that mimics old incandescent bulbs — the light most of us grew up with — and it's the temperature that makes warm tones sing. Reds, terracottas, creams, muddy pinks and earthy yellows all hold their glow under 2700K. Push up to 3000K and you'll start to lose a touch of that cosiness; go to 4000K or beyond (the cool, bluish stuff sold as "daylight") and you'll flatten the warmth completely, sometimes turning a lovely warm cream into something grey and dead.
Here's why it matters: warm paints are reflecting the long, red end of the spectrum. Feed them a cool, blue-heavy bulb and there's simply less warm light for them to bounce back. You've effectively cancelled out the very thing you paid for.
So if you've gone for something like Farrow & Ball Setting Plaster — that gorgeous dusky pink — or Little Greene's Masquerade, a deep warm terracotta, light them at 2700K and they'll feel rich and enveloping. Same with a soft cream like Edward Bulmer Jonquil or a warm off-white such as Farrow & Ball Slipper Satin: 2700K keeps them looking buttery rather than washed out.
"But what about north-facing rooms?" — the temptation is to whack in cooler bulbs to "brighten" a dim room. Don't. A north-facing room already gets cool, bluish daylight; a cool bulb just doubles down on the chill. If anything you want 2700K (or even 2400K in a snug or bedroom) to push back against it.
Two practical things. First, check the CRI — colour rendering index — and buy bulbs rated 90+. A cheap 2700K bulb with a CRI of 80 still makes colours look murky. Second, keep it consistent: mismatched temperatures across one room look dodgy. Buy a job lot of the same bulb and you're sorted.