Paint doesn't have a colour in any absolute sense — it has a colour under a given light. What you see is the light source bouncing off the pigment, and your eye reading whatever wavelengths come back. Change the light, change the colour. Simple as that.
Daylight is roughly even across the spectrum (cooler in the north, warmer at golden hour). Most home lighting at night is the opposite — warm white LEDs and old incandescent-style bulbs sit around 2700K, heavy on yellow and orange. So that colour pumps warmth into your paint after dark. Cool greys go beige or muddy. Soft whites turn cream. Greens lean yellow. It's not the paint misbehaving, mate — it's physics.
The effect is worst with colours that already sit on a knife-edge. Farrow & Ball Cornforth White is a classic offender — a balanced grey by day that warms up noticeably under lamplight. Ammonite does the same. If you want a grey that holds its nerve at night, you need one with enough blue or green backbone to resist the yellow shift — Little Greene French Grey - Pale stays calmer than most, and Farrow & Ball Pavilion Gray keeps its cool composure because the underlying blue counters the warm bulbs.
The "but what about" question: should you just buy cool-temperature bulbs? You can, but a 4000K bulb in a snug or bedroom feels clinical and unwelcoming. Better to choose the paint to suit how you actually live in the room — warm 2700K lighting and a colour that's designed to glow under it, like Farrow & Ball Setting Plaster, which only gets lovelier as the sun drops.
Practical advice: never sign off a colour from a daytime sample alone. Get a proper sample pot — or a peel-and-stick from COAT or Lick — and look at it on the wall at 8pm under your real lamps, lights on. The colour you live with at night is the one that matters most.