The Colour Room.

How it reasons

Most colour tools ask what you like. This one asks what your room is, because a colour does not behave the same way in two different rooms — and in London, where a Belgravia drawing room and a lower-ground kitchen sit in the same house, that difference is the whole job.

Aspect, and the light it lets in

North-facing rooms take cool, indirect light all day. It flatters blues and greys and drains the warmth from anything trying to be a warm white, which is why a north room is usually better served by a colour with a warm base, or by committing to real depth rather than fighting the light. South light stays generous and true from morning to dusk and will carry almost anything. West rooms turn golden late in the day, so a room used in the evening should be chosen for the evening. East rooms are at their best in the morning.

The compass is only half of it. A London street, a lightwell, a neighbouring wall or a tree in leaf can matter more than the direction the window faces — so the tool asks how much daylight actually arrives, and whether anything green sits close enough to throw a cast onto the walls.

Rooms with no window at all

An internal cloakroom or a windowless bathroom is a different problem entirely, and aspect is meaningless in it. Under artificial light, cool greys and neutrals tend to read flat and lifeless; the honest answers are warmth, or real depth used deliberately. In those rooms the tool says plainly that your lighting will do more than your paint, and that no colour can manufacture daylight.

What the room already contains

Floors, worktops, sanitaryware and large pieces of furniture are the fixed colours in a scheme; the walls have to agree with them. A cool grey against yellow-toned oak is the commonest mistake in the book. So the question changes with the room — a hallway is asked about its floor, a kitchen about its worktops and cabinetry, a bathroom about its tiles and fittings.

Plaster, damp and the right paint system

Most period London rooms have modern gypsum plaster and take ordinary emulsion perfectly well. Where a wall is genuinely lime-plastered, permeability matters and a washable, film-forming paint is the wrong system for it. Where there is damp, staining or mould, the source has to be identified before any decorative system is specified at all — paint is a finish, not a remedy.

The finish for each surface

Colour is only half a specification. Walls in a room you sit in want a chalky matt; a hallway wants something matt but scuff-resistant; a bathroom wants a moisture-resistant finish, and Little Greene recommend an eggshell on bathroom walls rather than a matt. Woodwork wants an eggshell, and kitchen cabinetry a higher sheen again. Every recommendation names the finish as well as the colour.

What the tool will not do

It will not tell you a colour is right because you asked for it. Ask for light and airy in a room with no window and it will say so. Its swatches are approximations, as every screen's are, and it ends by telling you to order sample pots and look at them in morning and evening light — which is what we would tell you standing in the room.