Why does my new paint colour look completely different on the wall?

Several effects stack up, and together they explain why a colour that looked perfect on a card can look unrecognisable once it is on four walls.

Light source is the biggest factor. Colour is reflected light, so the light doing the reflecting changes it. A paint viewed under warm 2,700K bulbs, cool daylight bulbs, and north-facing daylight is genuinely three different colours. North-facing rooms get cool, blue-tinted light all day, which pushes greys and whites cold and can make a warm beige look grubby. South-facing rooms get warm light that intensifies yellows and warms greys. This is why the shop's fluorescent lighting is a poor place to judge anything.

Scale changes perception. A colour covering a whole room reads considerably more saturated and darker than the same colour on a small card, because you are seeing far more of it and because light bounces between the walls, reinforcing the hue. A colour that looks like a hint of grey on a chip can look distinctly blue across a room. The usual correction is to choose a shade lighter than seems right.

Surrounding colours shift it. Simultaneous contrast means a colour appears to shift away from what is next to it — the same grey looks warmer beside blue and cooler beside beige. Flooring, furniture and curtains all pull at it.

Undertones surface at scale. Most neutrals carry a bias — green, pink, yellow or violet — invisible on a chip and obvious on a wall.

The fix is sample pots, not cards. Paint a large patch, ideally two coats on a piece of lining paper you can move around the room, and look at it in morning light, afternoon light and under your own bulbs at night before deciding.