Colour in Interiors: What Actually Happens When Light Hits Paint

Almost everyone has painted a room a colour they loved in the shop and disliked at home. The explanation is not taste. It is physics.

Paint does not have a colour — it has a response

A surface appears a given colour because it absorbs some wavelengths and reflects others. Change the light source and you change which wavelengths are available to reflect. A grey mixed under northern daylight can turn faintly lilac under warm domestic bulbs, because the blue it was relying on is not in the room.

When two colours match under one light and diverge under another, that is metamerism, and it is the single most common reason a sample disappoints.

Undertone is the thing to look for

Nearly every neutral leans somewhere — green, pink, yellow, blue or violet. On a small chip the lean is invisible; across a whole wall it dominates. The reliable test is comparison: place the candidate against a sheet of pure white paper and against two other neutrals, and the bias becomes obvious.

LRV tells you how the room will feel

Light Reflectance Value runs from 0 (absorbs everything) to 100 (reflects everything) and is printed on most professional fan decks. It predicts brightness better than the colour name does. Below roughly 25 a colour will consume light and make a small room read smaller; above roughly 70 it will bounce light and can glare in a south-facing room.

Orientation changes everything

North-facing rooms receive cool, steady, indirect light, which pushes colours toward grey and can make a cool paint look cold — warmer undertones compensate. South-facing rooms get warm, strong, variable light that can make a warm colour look brash by afternoon. East and west rooms shift dramatically between morning and evening, so judge them at the hour you use them.

How to test properly

Paint two coats on a large sheet of primed card, not directly on the wall where the existing colour contaminates the reading. Move it to each wall. Look at it in morning light, in afternoon light, and under the lamps you actually own. A colour that survives all three is the one to buy.