The short answer. For anything you sit in after dark, 2700K. For a kitchen or a mirror you actually use, 3000K. For a desk lamp you work under, 4000K. Ignore the word daylight on the box unless you are lighting a workshop, and check the colour rendering number before you check anything else.

I bought a pack of six bulbs once because they were cheap and the box said daylight, which sounded like the honest, neutral, correct choice. It was not. For about a year my flat had the ambience of a regional airport, and I could not work out why until a friend said, standing in the kitchen, that it felt like a place where you would be told off.

The bulb was 6500K. The room was 2700K worth of wood, wool and warm paint. They were fighting.

The number that matters is on the back

Bulb packaging leads with watts, which stopped meaning brightness about a decade ago, and hides the two numbers that actually decide how a room feels.

Kelvin (K) is the colour of the light. Lower is warmer, meaning more orange. Higher is cooler, meaning more blue. It is a temperature scale borrowed from how hot metal glows, which is why the numbers run the opposite way to intuition: 2700K is a candle-ish orange, 6500K is a midday sky.

CRI is colour rendering index, 0 to 100, and it is how faithfully the light shows colour. A bulb can be exactly the right temperature and still make everything look slightly wrong if the CRI is low. Anything at 90 or above is good. Below 80 and reds go brown, greens go grey, and people look faintly unwell.

A cheap bulb with 2700K and CRI 72 will look worse than a slightly more expensive 2700K at CRI 95, and the difference is not subtle. It is the difference between a room that looks furnished and a room that looks like a showroom.

What to put where

These are the numbers I have settled on after moving them around for a few years.

Bedroom: 2700K, low output. A bedroom wants the least light of any room in the house and the warmest. If you read in bed, a separate small lamp at 2700K rather than a brighter overhead. Nothing above 3000K belongs in a bedroom, including the bathroom light you see through an open door.

Living room: 2700K. Warm, and at two or three heights rather than one, which is a separate argument I make in the one-overhead-light problem. If the room doubles as a desk during the day, a 4000K task lamp that you switch on separately is better than making the whole room cool.

Kitchen: 3000K. You want to see what you are cutting. 2700K in a kitchen makes raw chicken look fine when it is not, and makes a stainless worktop look yellow. 3000K is warm enough to sit under at night and neutral enough to work under at seven in the morning.

Bathroom mirror: 3000K, and put it in front of your face. The most common bathroom lighting mistake is a single fitting on the ceiling behind you, which lights the top of your head and puts your face in shadow. A light at the mirror, at face height, at 3000K, is the fix. This is also why hotel bathrooms look better than home bathrooms.

Desk: 4000K. Cool, and only over the desk. 4000K is the temperature of a bright office and it is genuinely easier to concentrate under, which is exactly why you do not want it over the sofa. A desk lamp at 4000K in a 2700K room works because the pool of light is local. A ceiling at 4000K in the same room does not.

Hallway and stairs: 2700K, and not much of it. A hallway is a place you pass through. Bright cool light in a hallway is what makes a flat feel like a corridor between two rooms rather than part of the home.

The two mistakes people actually make

Buying daylight because the word sounds neutral. Daylight on a package is marketing, not a specification. It usually means 5000K to 6500K, which is the colour of the sky on a grey day — literally the light you complain about in February. The neutral-sounding option is the one that makes a room feel coldest.

Mixing temperatures in one sightline. You put a 2700K bulb in the table lamp and leave the 4000K bulb that came with the flat in the ceiling fitting, and now the room has two colours in it. Almost nobody can name what is wrong, but everybody notices. Pick one temperature per room, and if you cannot change the ceiling fitting, do not use it at the same time as the lamps.

There is a third, subtler mistake: buying a bulb by wattage. A 9W LED and a 15W LED can both be 800 lumens. Lumens is brightness. Watts is what it costs to run. Read the lumens.

Fittings you cannot change

Rentals, and most flats built before about 1990, come with fittings that take one kind of bulb and a switch that does one thing. You still have options, and they are cheaper than an electrician.

A screw-in adapter. An E14 fitting can take an E27 bulb through a two-pound adapter, which opens up every bulb in the shop. It looks slightly odd and nobody has ever noticed.

A shade instead of a bulb. If a fitting is too bright, the fix is often a shade that covers more of the bulb rather than a dimmer bulb. A brighter bulb behind a wider shade is a better light than a dim bulb with nothing on it.

One lamp with a dimmer, instead of a dimmable ceiling. This is the whole strategy of the piece on lamps at three heights: make the ceiling light the thing you use for cleaning, and put the light you actually live under on a plug-in dimmer.

A smart bulb in the one fitting you cannot reach. For a hallway or a stair light with a switch at the bottom and no second switch at the top, a cheap smart bulb with a phone control is genuinely useful, and it is the only place in a flat where I would bother.

What I would not do is replace every bulb in a rental with a smart one. The fittings are usually the wrong shape, the app is usually worse than a switch, and you will take them with you when you leave.

If you want to change the temperature by time of day

The honest version of the tunable-white pitch is this: a room used for work in the afternoon and rest in the evening genuinely benefits from going from 4000K to 2700K as the day ends. Whether that is worth a smart bulb depends on how much you dislike apps.

Two things I would say from having tried it. First, most tunable bulbs are dimmer at the warm end than a plain warm bulb, so the room gets darker as it gets warmer, which is either the point or a problem depending on the room. Second, the automation is the fragile part. A bulb that needs the router is a bulb that will be at full brightness at 2am the first time the router reboots.

The low-tech version — one warm bulb, one dimmer, one switch you turn off — gets you most of the way and never updates itself into a worse state.

The short version, if you are standing in a shop

For the rooms you sit in: 2700K, CRI 90 or above, dimmable, 800 lumens. For the kitchen and the bathroom mirror: 3000K, same everything else. For a desk: 4000K.

That is the whole decision. The rest of the packaging is there to make you buy six of the wrong ones.