Every colour sits somewhere on a warm-to-cool scale, driven by its undertone — the subtle bias hiding underneath the main colour you see on the tin.
Warm colours carry red, yellow or orange in them. They advance, feel cosy, and make a space feel smaller and more enveloping. Think Farrow & Ball Setting Plaster (a pink-led plaster tone) or Slipper Satin, a creamy off-white with a yellow heart.
Cool colours carry blue, green or violet undertones. They recede, feel fresh and airy, and can read clinical if you push them too far. Farrow & Ball Wevet and Strong White both have a cool grey bias — crisp and gallery-like in the right light.
Here's the bit that catches people out: a colour's temperature isn't fixed by its name. A "grey" can be warm (Little Greene French Grey, which has green-yellow in it) or cool (a blue-grey like F&B Pavilion Gray). That hidden undertone is why two greys that look identical in the shop can fight in your hallway.
Light matters enormously. North-facing rooms get cool, bluish daylight that drains warmth and exaggerates anything cold — so a cool grey there can turn frankly miserable. South-facing rooms get warm light that flatters almost everything. The rule of thumb: warm up north-facing rooms, and you can get away with cooler tones in south-facing ones.
The most common "but what about" — *why does my white look blue/yellow?* That's the undertone showing under your lighting. Cool whites like Wevet go icy under LED; warm whites like Slipper Satin can go custardy under warm bulbs.
Practical advice: never judge temperature off a website swatch. Get a peel-and-stick sample or paint a board, move it round the room over a full day, and watch how it shifts morning to evening. If it feels cold and unwelcoming at 7pm, it's too cool for that space. Trust your gut — your eye reads temperature before your brain names the colour.