What does 2700K mean on a light bulb?

It means the light is the colour a piece of metal would glow at 2700 kelvin, which is a warm yellowish white. It is the standard domestic warm bulb and the right default for living rooms and bedrooms. Lower numbers are more orange, higher numbers more blue.

Is warm white or cool white better for a dark room?

Warm, in the 2200K to 2700K range, and the same number on every bulb in the room. Dark walls return very little light, so there is nothing to average out a mismatch, and a single cooler bulb among warm ones looks like a broken fitting rather than a choice.

Why do my LED bulbs look cold when I dim them?

Because most LEDs lose brightness without changing colour, unlike a filament, which gets warmer as it cools. Look for bulbs sold as warm dim or dim to warm, which shift from around 2700K down to about 1800K as they go down.

What CRI should I look for?

90 or above if it is quoted, especially in a dark room. Below about 85, dark colours drift towards grey and brass and wood look flat, because there is less reflected light available to make up the difference.

Dark interiors

Colour temperature in kelvin, in plain words

3 November 2026 · 5 min read

Kelvin is the number on a bulb box that decides whether a room looks like a living room or a corridor in a hospital. It runs backwards from how most people expect: the low numbers are warm and orange, the high numbers are cold and blue. 2700K is an ordinary warm bulb, 4000K is office light, 6500K is overcast daylight. That is most of what you need, but the reason behind the scale is worth five minutes, because it explains why one wrong bulb in a room is so noticeable.

Why the scale runs backwards

Heat a piece of iron and it glows red, then orange, then yellow, then white, then faintly blue. Physicists describe that sequence by the temperature of the metal in kelvin, which is the same as degrees Celsius with 273 added on. A dull red glow is around 1000K. An old tungsten filament ran about 2700K. The surface of the sun is about 5800K.

So the number on a bulb is not the temperature of the bulb. It is the temperature a piece of hot metal would have to reach to glow that colour. A 2700K LED does not get anywhere near 2700 degrees, it just produces light the colour that a filament at that temperature would.

That is why it feels inverted. The colours we call warm come from genuinely cooler objects: a candle flame, an ember, a filament at the end of a dimmer. The colours we call cool come from much hotter sources, which is a quirk of language rather than physics.

The numbers, and where each one belongs

1800K to 2200K is candlelight and the deep amber end of filament style bulbs. It is beautiful and it is almost useless for seeing anything in detail, so it belongs in a decorative fitting, a lamp you look at rather than read by, or a dimmed source late in the evening.

2700K is the standard warm white, and the one to default to for living rooms, bedrooms and hallways. 3000K is a half step cooler, slightly crisper, and suits a kitchen or a bathroom where you want to see what you are doing without the room feeling clinical.

4000K is neutral white, which is office and workshop light. In a house it reads as cold and institutional in any room with soft furnishings in it, and it is the temperature most frequently bought by accident, because the boxes are hard to read and because 4000K bulbs are usually the cheap ones on the end of the shelf.

5000K to 6500K is daylight, and it has two honest uses at home: a task lamp for close work where colour accuracy matters, such as matching thread or paint, and a garage. In a living room it looks like a strip light in a shop.

Mixing temperatures is the mistake that reads as a fault

Your eye adapts to the average colour of the light in a scene and then stops noticing it, which is why a 2700K room looks white rather than orange after a few minutes. What your eye will not do is adapt to two temperatures at once. Put one 4000K bulb among four 2700K bulbs and the cool one looks distinctly blue and slightly wrong, like a fitting that has developed a fault.

This gets worse in a dark room. With dark walls there is very little reflected light to average out the difference, so each source keeps its own colour and the mismatch stays visible all evening rather than fading.

So the cheapest single improvement available in most houses is replacing the odd bulbs out. Go round with a notebook, note what is in each fitting, and standardise the room. It is more effective than buying another lamp, and it costs the price of two bulbs.

Dimming changes the colour, or it should

A filament bulb gets warmer in colour as it dims, because you are literally cooling the filament. That is the behaviour everybody has internalised from a lifetime of incandescent light, and it is why a dim room is expected to be an orange room.

Most LEDs do not do this. Dim a standard LED and you get less light at exactly the same colour, which looks grey and slightly dead rather than cosy. The feature that restores the old behaviour is called warm dim or dim to warm, and a bulb with it will typically slide from 2700K down to around 1800K as it dims.

It costs a little more and it is worth it in any room you sit in at night. If you are buying bulbs for a dark room anyway, this is the specification to look for before brightness, because the usual complaint about a dim dark room is not that it is too dark but that it looks cold.

Colour rendering is the other number, and it matters more here

CRI, the colour rendering index, is a score out of 100 for how faithfully a light source shows colour compared with daylight. Cheap LEDs sit around 80, which is adequate for a corridor. 90 or above is where dark colours stop collapsing towards grey.

This matters far more in a dark room than in a pale one, because there is less light bouncing around and therefore less information for your eye to work with. Under a low CRI bulb a dark red reads brown, a deep green reads black, and brass looks like dull yellow plastic. Under a 90 plus bulb at the same brightness the same objects separate properly.

It is especially visible on metal. The reason brass turns up so often in dark rooms is that it is one of the few things that returns light at all, and a bulb that renders colour badly throws that away. A cast brass piece or a pair of black iron candlesticks on a dark surface will look like two different objects under a cheap bulb and a good one.

Reading a bulb box without being misled

Four numbers are worth finding. Kelvin, which you now know. Lumens, which is actual light output and the only honest measure of brightness, since watt equivalents are marketing. CRI or Ra, where quoted. And the beam angle if it is a spot rather than a bulb.

Watch out for names rather than numbers. Soft white, warm white and cool white are not standardised across manufacturers, and two bulbs labelled warm white from different brands can be 300K apart, which is enough to see. If the box gives a name and no number, buy a different bulb.

Be slightly sceptical of filament style bulbs with a visible spiral, because the thing that makes them attractive, a bright visible source, is the thing that causes glare in a dark room. In a dark scheme a clear filament bulb in a shadeless fitting at eye level will spoil your night adaptation every time you look at it. Keep the look and put it inside a shade, or up high, or on a dimmer run low.

Questions people actually ask

What does 2700K mean on a light bulb?: It means the light is the colour a piece of metal would glow at 2700 kelvin, which is a warm yellowish white. It is the standard domestic warm bulb and the right default for living rooms and bedrooms. Lower numbers are more orange, higher numbers more blue.
Is warm white or cool white better for a dark room?: Warm, in the 2200K to 2700K range, and the same number on every bulb in the room. Dark walls return very little light, so there is nothing to average out a mismatch, and a single cooler bulb among warm ones looks like a broken fitting rather than a choice.
Why do my LED bulbs look cold when I dim them?: Because most LEDs lose brightness without changing colour, unlike a filament, which gets warmer as it cools. Look for bulbs sold as warm dim or dim to warm, which shift from around 2700K down to about 1800K as they go down.
What CRI should I look for?: 90 or above if it is quoted, especially in a dark room. Below about 85, dark colours drift towards grey and brass and wood look flat, because there is less reflected light available to make up the difference.