You pick a soft sage from a tiny card, paint the wall, and it looks grey in the morning and almost yellow by evening. Nothing is wrong with the paint. A colour is never just the pigment: it is the pigment and the light landing on it. Change the light and the same wall gives your eye a different message.

Slide the day across your wall

Pick a paint colour, choose which way your window faces, then slide the time of day. The small swatch is the colour as it looks in neutral light, like the card in the shop. The big panel is the same colour on a wall in your room.

Why the same paint changes

Paint does not make light; it only reflects some of the light that reaches it, and it reflects some colours more than others. A sage wall bounces back plenty of green and less of the rest. If the light carries little green, or lots of orange, the wall has less to work with and your eye reads something new.

Light has its own colour, usually described as a temperature in kelvin. Low numbers (around 2700 K, a warm bulb or a low sun) are yellow-orange. High numbers (6000 K and up, an overcast sky or a north window) are bluer. The same paint under 2700 K and under a grey sky really does send different light to your eye.

What each window direction tends to do

These are tendencies in the northern hemisphere, and they depend on trees, neighbours and how high your floor is.

• North-facing: no direct sun, so the light is even, steady and a little cool. Cool greys and blues can feel chilly; warmer, slightly creamy colours often feel friendlier.
• East-facing: bright and warm in the morning, then cooler and softer once the sun moves round.
• South-facing: the brightest, with strong light for much of the day. It copes with most colours, but very yellow ones can feel heavy.
• West-facing: quiet in the morning, golden and strong in the late afternoon and evening.

Evenings are lamp time

Most of us sit in our living rooms after dark, so a wall's evening colour matters as much as its noon colour. A warm bulb adds yellow and takes away blue, which is why blue-greys often look greyer and whites look creamier at night. Check your colour under the bulbs you actually use.

Test before you commit

You do not need to buy more paint to be sure; you need to look properly once.

1. Paint a large sample, about A3 size, on lining paper or a spare card, rather than straight onto the wall. Two coats, so the old colour does not show through.
2. Move it around: stick it on the wall you plan to paint, and on a different wall too. Light bounces, so each wall sees a different room.
3. Look at it in the morning, at noon, in the evening and under your lamps, over a day or two.
4. Put it next to your floor, curtains and sofa. The things beside a colour change how it looks.

If you rent and cannot paint, the same idea works in reverse: change the light instead. A warmer bulb, a lamp in a dark corner or a pale rug can shift how the walls feel without touching them.

When to break the rule

A common tip says to pick warm colours for north rooms and anything for south rooms. It is a decent starting point, not a law. Some people love a cool, grey-blue north room because it feels calm and cave-like, and a deep colour can make a dim room feel deliberate rather than dark. If you like the result in your own test, trust it.