Warm vs cool colours, and why it's relative

Orange is the warmest colour and blue is the coolest. People across cultures agree on that much. Everything in between, though, is a comparison. Green and purple sit right on the boundary, and researchers disagree about exactly where it falls. That's why the more useful question isn't "is this colour warm?" but "is it warmer than the one beside it?"

What makes a colour warm or cool?

There really is a warm end and a cool end. In a 2025 study, researchers asked 25 people to rate colours from warm to cool, and nineteen of them sorted every hue along the same single line, from orange-red at the warm end to greenish-blue at the cool end. An earlier experiment by a different team found the same two ends: warmest at orange, coolest at blue.

Now, that line doesn't match the red-versus-green or blue-versus-yellow pairs you might have learnt in colour theory. It cuts across both of them. And not everyone sees it. Six of the 25 people rated colours in a pattern that didn't follow a warm-to-cool line at all.

Which hues feel warm is something cultures share. A 2025 study of children and adults in several regions found that reds and yellows read warm and blues read cool everywhere they looked. What changed from culture to culture was how much lightness and saturation mattered, and the link between colour and temperature grew stronger with age.

It's also more than a word. People find it easier to tell two colours apart when one is warm and the other is cool than when both sit on the same side.

A side note on light bulbs: "warm white" light has the lower number. Bulbs are rated by the temperature of an ideal glowing object that gives off the same colour of light. Heat a poker and it glows red, then orange, then yellow and white. Keep heating it and it turns bluish. So a "cool white" 6500K bulb matches a hotter object than a "warm white" 2700K one. The labels describe how the light feels, not how hot anything is.

Where is the line between warm and cool colours?

Nobody has found a fixed one. The studies agree on the ends, but they disagree about the middle. When people do give an in-between answer, which is rare, it clusters mostly around green and around purple.

So orange and blue are settled, and green and purple are where the argument happens.

The disagreement isn't new, either. The colour educator David Briggs points out that writers have put the boundary anywhere from inside red and green to inside yellow and violet. In the 2025 study, the warmest and coolest colours held steady when lightness changed. The green-side boundary didn't.

Warm and cool on your hue slider

I mapped the research onto the hue slider in your design app, at full saturation and full brightness. Min & Park's 2024 study is the only one that gives its boundary as a number I can convert, so the green side rests on it. The purple side shows where the different placements disagree.

Pure screen green (#00FF00, hue 120) sits just short of the measured line. It's about 16 slider degrees away, which makes it borderline rather than warm. Keep in mind the line was measured on duller, darker colours, and the green side is the least stable part of it, so the vivid end of the slider is an estimate.

Screen magenta (#FF00FF) and CSS purple (#800080) land in the disputed zone. One placement calls them cool and another calls them slightly warm. Bluer purples, such as blue-violet, rebeccapurple and indigo, come out cool on every placement. And yellow-green and chartreuse come out warm on every measure, which surprises most people who file all greens as cool.

That matters if your palette leans on a bright green or a magenta as its "cool" colour, because those are the two the research can't settle.

Why can the same colour be warm or cool?

Because warmth is a lean, and every hue can lean both ways. A colour only counts as warm or cool against something else. Think of height. A 175cm woman is tall in a primary school classroom and average on a netball court. She hasn't changed at all. The comparison has.

The painters who gave us these words already treated warmth this way. Back in 1758, Robert Dossie defined warmth as a lean within a colour, not a family of colours. To him a warm red leaned towards orange, a warm yellow towards red, and a warm blue towards purple. A cool yellow or blue leaned towards green. Even then he grumbled that painters used the words "very indefinitely".

Josef Albers took it further in Interaction of Color (1963). He went along with the Western habit of calling blue cool and yellow, orange and red warm, but he argued that, like any temperature, these readings are "only relative". Which means warm blues and cool reds both exist. Johannes Itten, whose colour wheel most design courses still teach, made the same point in his figure captions: red-violet looks warm next to blue and cold next to orange.

In this comparison the colour itself doesn't change. What changes is your verdict on it. A neighbour can also change how a colour looks, but that's a different effect, and it has its own article.

The order you see colours in can move the line

In one small study, it did. Min and Park, two researchers in Seoul, showed people colours one at a time, working their way round the colour wheel, and asked when each one switched from warm to cool. People who travelled round the wheel switched sooner than people who saw the same colours in a random order. That held whichever direction they went, so the line landed in different places depending on which way they were travelling.

Orange and blue stayed put whatever the order. The regions that moved were yellow-to-green and purple-to-red. The authors describe warmth as having "constant and variable sections". Only 10 people took part in that part of the study, though, so treat it as a lead rather than a settled fact.

Warm things tend to be the subject

Look out of a window. The sky, the hills, the water and the shade run blue, grey and green, but fruit, flowers, skin and fire run red, orange and yellow. A 2017 study in PNAS looked at about 20,000 photographs and found the same thing: objects tended to be warm-coloured and backgrounds cool-coloured.

The same study covered 110 languages. In every one of them, people described warm colours more precisely than cool ones, and the researchers argue that's because warm things are usually what gets talked about.

A warm accent on a cool background matches how the world tends to look.

Do warm colours advance and cool colours recede?

Sometimes, and it depends on the background. Your eye bends red and blue light by slightly different amounts, so most people see red as a little nearer than blue. But in a 1995 lab study, the effect reversed when the background colour changed, and the same target could sit at different depths on different backgrounds.

So "warm advances" is relative too. Albers doubted it as a rule, and he pointed to painters who used a deep vermilion that sits back.

Do warm colours make a room feel warmer? Only a little. The famous claim comes from Itten, who said a blue-green workroom felt cold at 15°C and a red-orange one didn't until about 11 or 12°C. He gave no study for it. Research on coloured lighting finds bluish light makes people feel slightly cooler, mostly in the first half-hour. I found no study on wall paint.

What should you do differently in your next design?

Stop sorting colours into two fixed lists, and judge each one against its neighbours instead.

1. Ask "warmer than what?" Before you call a colour warm, name the colour it sits beside. The answer can flip when the neighbour changes.
2. Name the lean, not the temperature. Briggs suggests saying "reddish blue" rather than "warm blue". That way a client, a printer or your future self will know exactly what you meant.
3. Test green and purple in place. If a palette relies on green or purple as its "cool" colour, check it beside the colours it will actually sit next to. Bright green and magenta are the least reliable on the slider.
4. Give warm the subject and cool the ground. That's my rule of thumb, not a law. It follows the pattern in real photos, and it's a sensible place to start when you need one element to be noticed.
5. Check your neutrals side by side. A true grey is neither warm nor cool. But most "greys" and "whites" in a palette carry a slight tint, and when you put two next to each other, the lean shows.

Frequently asked questions

Is purple warm or cool?

Purple sits on the line between warm and cool, which is why sources disagree about it. When people rate colours from warm to cool, the switch from cool back to warm happens around purple, but different studies place it in different spots. Bluish purples such as indigo read cool, and reddish purples and magentas are disputed. Next to blue, a reddish purple looks warm. Next to red, it looks cool.

Is green warm or cool?

Green is borderline. Most people call bluish greens cool, but the measured switch from warm to cool falls somewhere between yellow-green and green itself, and the spot changes from study to study. In my measurements, pure screen green (#00FF00) sits just short of the line. So a yellowish green reads warm beside blue, and a bluish green reads cool beside orange.

Can a red be cool, or a blue be warm?

Yes. Warm and cool describe which way a colour leans, and every hue can lean both ways. A red that leans towards orange is warm, and one that leans towards purple is cool. Robert Dossie's 1758 painter's manual, The Handmaid to the Arts, defined a warm blue as one leaning slightly towards purple. Josef Albers later called these judgements "only relative", so warm blues and cool reds both exist.

Why is "warm" light a lower colour temperature than "cool" light?

Because the two uses of "warm" run in opposite directions. Colour temperature is the temperature an ideal glowing object would need to give off light of that colour, and hotter objects glow bluer. So bluish "cool white" light at about 6500K matches a hotter source than orange "warm white" at about 2700K. The name "warm" comes from how the light feels, like firelight, not from the physics.

Is grey (or white) warm or cool?

A truly neutral grey or white has no hue, so it's neither warm nor cool. Plenty of greys and whites in real palettes carry a slight tint, though, and that tint leans them one way. A grey that leans towards yellow or red reads warmer. One that leans towards blue or green reads cooler. Put two "neutral" greys side by side and you'll see which way each one leans.

For the curious

The warm-to-cool line in research is usually described in CIELAB, a colour space built around how people see. The standard colour-emotion model (Ou and colleagues, 2004) points its warm-to-cool axis along a hue angle of 50°, an orange-red. I haven't read that paper's full equation, so I only use its direction, to rank colours against each other. Min and Park report their warm/cool conversion points at CIELAB hue angles of about 140–150° and 335–360°.

My slider mapping converts sRGB to CIELAB, and I checked it against a second, independent library (colour-science). Pure screen green comes out at a hue angle of 136°. The way people tell colours apart more easily across the warm/cool line is called categorical perception (Holmes and Regier, 2017). Brighter versions of a colour are also judged warmer (Hammond and colleagues, 2024).

Light bulb ratings use correlated colour temperature, measured against an ideal Planckian radiator. Hotter objects glow bluer because of Wien's displacement law. And the advancing-red effect is called chromostereopsis, which is caused by chromatic aberration in the eye.

Sources (14)

• Jake Manalansan, Lorne A. Whitehead and Michael A. Webster, "Warm versus cool colors and their relation to color perception" (opens in a new tab), Journal of Vision 25(4), 2025
• Jihae Min and YungKyung Park, "Order Effect of Warm-Cool Judgment of Colors" (opens in a new tab), Archives of Design Research 37(4), 2024
• Kenneth Knoblauch, John S. Werner and Michael A. Webster, "Warm and cool reheated" (opens in a new tab), Color Research & Application 48(6), 2023
• Edward Gibson and colleagues, "Color naming across languages reflects color use" (opens in a new tab), PNAS 114(40), 2017
• Robert Dossie, The Handmaid to the Arts (opens in a new tab), vol. I, 1758
• Josef Albers, Interaction of Color, Yale University Press, 1963
• Johannes Itten, The Art of Color, 1961, and The Elements of Color, ed. Faber Birren, 1970
• David Briggs, "The Dimensions of Colour: warm and cool hues" (opens in a new tab)
• Barry Winn and colleagues, "Reversals of the Colour-Depth Illusion Explained by Ocular Chromatic Aberration" (opens in a new tab), Vision Research 35(19), 1995
• Hsin-Ni Ho and colleagues, "Combining colour and temperature" (opens in a new tab), Scientific Reports 4, 2014
• CIE, colour temperature, term 17-23-067 (opens in a new tab), and CIE TN 013:2022 (opens in a new tab)
• Li-Chen Ou and colleagues, "A study of colour emotion and colour preference. Part I", Color Research & Application 29(3), 2004
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• Ndeye Meissa Koura Sow and colleagues, "A Cross-Cultural and Developmental Investigation of the Association Between Color and Temperature", Color Research & Application 51(1), 2025