Whites are never just white — they all carry an undertone, and you only spot it when you put them next to something. On its own, every white looks white. The undertone is the faint colour mixed in to soften the brilliant white base, and that's what dictates whether a room feels cosy or clinical.
The reliable test: brush two or three whites onto a sheet of white card, leave a gap between them, and stand back. The undertones will jump out by comparison. A warm white reveals a hint of yellow, butter or pink. A cool white shows grey, blue or green. Do this against a genuinely pure white — printer paper is a decent reference point.
Farrow & Ball's Pointing is a textbook warm white — soft, creamy, a touch of yellow that makes it feel gentle and lived-in. Their Wevet sits cooler, with a whisper of grey that keeps it crisp without going stark. Strong White is the in-betweener — a cool, slightly grey-green white that behaves beautifully in modern spaces. Little Greene's Loft White leans warm and chalky, while Dulux's brilliant whites tend to read cool and blue.
Light matters enormously here. A north-facing room strips warmth out of everything and exaggerates cool undertones, so a cool white can tip genuinely grey and miserable — that's where a warm white earns its keep. A south-facing room floods with warm light and can make even a neutral white glow yellow, so you've more freedom to go cool.
The "but what about my trim?" question: if your woodwork white is cooler than your wall white, the contrast can look dirty rather than crisp. Keep undertones in the same family across a room and it'll feel cohesive.
Don't trust the name or the chip. Get sample pots, paint them big, and live with them across a full day. The undertone you couldn't see in the shop will be obvious on your own wall by teatime.