Light is the whole story here, mate. Paint doesn't have one fixed colour — it has a *response* to whatever light hits it. A shop is lit with bright, flat, usually cool overhead LEDs or fluorescents at high lux levels, often 4000K or higher. Your home is a different planet: warmer bulbs (2700–3000K), daylight pouring in from one direction, and bounce-back from wooden floors, carpets, even the neighbour's red brick wall opposite. All of that shifts how a colour looks.
The effect is strongest with greys, off-whites and anything with a complex undertone. Farrow & Ball Cornforth White is the classic offender — it leans clean and neutral under shop lighting, then turns soft and slightly mauve-pink in a north-facing room at home. Same paint, different light. Little Greene French Grey does the same dance, drifting greener or warmer depending on the room.
Then there's metamerism: two colours that match under one light source can look noticeably different under another. That little laminated swatch card was never going to behave the same on your wall.
The "but what about the tin lid?" question — no, the lid and the card both lie to you, because neither is in your room under your light. Here's what actually works:
- Get a proper sample pot. F&B, Little Greene, Mylands and most supported brands do them. Paint two coats on A4 paper or lining paper, not straight on the wall where the old colour shows through.
- Move it round the room — by the window, in the darkest corner, near the floor.
- Look morning, midday and night, with your actual lamps on. A colour like Farrow & Ball Pavilion Gray can feel crisp by day and properly moody under warm evening light.
Never, ever pick from the shop card alone. Test in situ, live with it for a couple of days, then commit. That's the only way to get it sorted.