At the simplest level: warm colours sit on the red-yellow-orange side of the wheel, cool colours on the blue-green-violet side. Warm tones advance and wrap a room around you — think of a soft terracotta or a buttery plaster. Cool tones recede, opening a space up and making it feel calmer and cleaner.
But the real trade secret is undertone, and that's where most people come unstuck. A grey isn't just grey — it's either a warm grey leaning toward brown or yellow, or a cool grey leaning blue or green. Farrow & Ball's Cornforth White has a cool, faintly mauve undertone, which is why it can read chilly in a north-facing room. Sit it next to Skimming Stone, which has a warmer, putty base, and you'll see the difference instantly. Same job, completely different mood.
Whites are the classic battleground too. F&B Wevet and Strong White lean cool and crisp; Pointing and Slipper Satin are warmer, with a creamy softness that flatters older houses.
Here's the bit that actually matters for your room: match the temperature to the light. North-facing rooms get cool, bluish daylight, so a warm colour balances it and stops the space feeling like a fridge — that's why a warm plaster like Setting Plaster works so well up north. South-facing rooms flooded with warm light can take a cooler colour without going cold.
The "but what about" question I always get: can you mix warm and cool? Absolutely — most good schemes do. A cool blue-green like F&B Treron on the walls with warm brass and timber stops it feeling clinical. It's the contrast that gives a room life.
Practical advice: never judge undertone from the tin or a website. Get a proper sample, paint a big patch, and look at it morning and evening. The temperature will shift through the day, and that shift tells you everything.