How does a color become "warm"?

Short answer

Red, orange, and yellow being considered "warm," and blue and cyan being considered "cool," comes largely from association: fire, sun, and embers sit on one side, water, ice, and shadow on the other; these associations are surprisingly consistent among people from different countries, but the color of a room doesn't measurably change how warm a person feels. Green, though, sits on the border between the two groups: greens leaning toward yellow are perceived as warm, those leaning toward blue as cool, and the same green shifts to one side or the other depending on the colors next to it.

Fire and water associations

When children are told to "show a warm color," most reach for red, orange, or yellow; told to show "cool," they reach for blue. This pairing is a relationship that everyday experience has built up over centuries. A flame's fire, the red glow of embers, the midday sun, and red-hot metal are examples of the warm side; the sea, ice, snow shadow, and a clear sky are examples of the cool side.

This distinction isn't just intuition; it's also measured in experiments. In a study published in 2004, thirty-one observers, fourteen British and seventeen Chinese, rated twenty colors on ten emotion scales such as warm-cool, heavy-light, and active-passive. The researchers found that these judgments clustered into three main factors: the color's activity, weight, and heat. The models they built for four color emotions, including the warm-cool judgment, agreed with the models from studies done in other countries; the authors suggested that these emotions appear to be independent of culture across countries.

This consistency shows how widespread an association is, but it doesn't mean the association changes the body. What color-emotion relationships generally do and don't find is discussed in PS-02.

Does color actually change heat?

The claim that "a red room feels warmer" has been debated for more than a century under the name hue-heat hypothesis. If the claim were true, painting a cold room in warm colors could reduce the need for heating.

An early and frequently cited study testing this claim was published in 1972. Twenty-one students, wearing red, blue, and clear goggles, rated their thermal comfort across twenty-minute sessions in which the wall temperature was varied between roughly 16 and 38 degrees Celsius. The effect of wall temperature and of differences between individuals was significant; the color of the goggles had no effect at all. The authors concluded that color produced a "purely mental" effect — that is, it could lead a person to believe they were warmer or cooler — but did not change their thermal comfort.

Later studies report mixed results; some find small effects, others find none at all. The safest thing to say today is this: color's effect on the sensation of heat, if it exists at all, is very small next to the effect of room temperature.

Depth in space: advancing and receding

Painters and interior designers say that warm colors advance and cool colors recede. This claim has two different roots.

The first root is optical. The eye's lens refracts different wavelengths slightly differently; short-wavelength blue refracts more strongly than long-wavelength red. To many people, red and blue patches placed side by side on a black background appear to sit at different depths. This effect is known as chromostereopsis, and its first scientific explanation belongs to Willem Einthoven in 1885.

But this effect doesn't run in the same direction for everyone. Einthoven proposed that, depending on the position of the pupil relative to the eye's optical axis, some people see red in front of blue and others see it behind. Today it's also known that most people see red as advancing, but that some see the reverse or no depth at all. So the rule "warm colors advance" isn't an optical law that holds for everyone — it's a tendency that holds for the majority.

The second root is observational. Distant mountains, hills, and buildings look pale and bluish because the air between them scatters light; this is the landscape-scale result of the scattering KE-02 describes. Painters have used this aerial perspective for centuries, making distant planes bluer and paler. The eye, too, has learned from this experience to conclude that whatever looks bluish and pale is far away.

The Kelvin paradox

When it comes to light, the words flip. An incandescent bulb's light is sold as "warm white," with a color temperature of about 2700 kelvin; a bluish sky light is considered "cool," but its color temperature is much higher. In physics, a hotter object emits bluer light; in perception, blue is cool. The physics of this contradiction, and the kelvin scale itself, are the subject of BI-07. What matters here is that the word "warm," for color, isn't a measure of temperature — it's the name of an association.

Is green warm or cool?

The question on the lesson card touches the weakest point of the warm-cool distinction. One half of the color wheel holds red, orange, and yellow; the other half holds blue and cyan; green sits exactly between the two.

So green's temperature depends on which green it is. A pistachio green or olive green leaning toward yellow sits close to the warm side; it calls to mind a sunlit meadow. An emerald green or sea green leaning toward blue sits close to the cool side; it calls to mind a shaded lakeshore. For a green sitting exactly in the middle, the call depends on its surroundings: it looks cool next to red and orange, warm next to blue and cyan. Because each of the color wheels covered in DT-04 places green in a slightly different spot, the "warm-cool" boundary also shifts from wheel to wheel.

The same is true for purple: a purple leaning toward red is considered warm, one leaning toward blue is considered cool. Warm-cool isn't a binary; it's a spectrum between two extremes.

Opposing view

Those who defend the warm-cool distinction point out that it's extremely functional in design: it has been used successfully for centuries to make an interior look more inviting or more spacious, to separate a painting's foreground from its background, and to give a brand an air of energy or calm. The fact that the associations turn out consistent across countries also shows that the distinction isn't an arbitrary fashion.

Those who are skeptical of the distinction emphasize its limits. There's no solid evidence that color changes the sensation of heat; the advancing effect doesn't run in the same direction for everyone; and the class of colors like green and purple changes with context. A color's "warmth," moreover, depends not only on its hue but also on its lightness and saturation: a pale pink isn't perceived as warm as a saturated red. On this view, warm-cool is a useful but rough shorthand, not a precise classification.

Example — "warm undertone" and "cool undertone"

In makeup and fashion, skin tones are often divided into "warm undertone" or "cool undertone." A warm-undertone skin is described as one that leans toward yellow and gold, a cool-undertone skin as one that leans toward pink and blue; this distinction is used for both light and dark skin tones.

What this lesson has shown helps you read these terms correctly. Human skin, from the lightest to the darkest, clusters within a small section of the color wheel's orange slice, spilling slightly into red and yellow; so in an absolute sense, no skin is in the "cool" color group. What's called "cool undertone" is the end of that narrow band closer to red and pink; "warm undertone" is the end closer to yellow. Warm and cool are relative words here too, just as with green. What's more, the same skin looks warmer under incandescent light and cooler under shaded sky light. Judging a person's undertone requires seeing them in neutral light against a neutral background; this is a result of the effect of surroundings covered in BI-14.

Common misconception

Summary

1. The warm-cool distinction rests on associations with fire, the sun, water, and ice; it's measurable in experiments and comes out consistent among observers from different countries, but there's no solid evidence that color changes the sensation of heat.
2. Warm colors advancing comes partly from chromatic aberration in the eye and partly from learning that what's far away looks bluish; the effect doesn't run in the same direction for everyone. In light, the words "warm" and "cool" contradict the kelvin scale.
3. Green and purple sit on the border and look warm or cool depending on context; the "warm" and "cool undertone" of skin are likewise relative words describing the two edges of the orange slice.

Test yourself

1. (Easy) What associations are warm colors linked to?
2. (Easy) What does the hue-heat hypothesis claim?
3. (Medium) Is green warm or cool?
4. (Medium) What did the 1972 goggle experiment find?
5. (Hard) Why isn't the rule "warm colors advance" a law that holds for everyone?
6. (Hard) If every skin tone sits in the warm region of the color wheel, what does "cool-undertone skin" mean?

Answers

1. With sources of heat such as fire, embers, the sun, and red-hot metal ("Fire and water associations").
2. That an environment whose dominant light is close to red feels warmer, and one close to blue feels cooler ("Does color actually change heat?").
3. It depends on which green it is and its surroundings. Greens leaning toward yellow are perceived as warm, those leaning toward blue as cool; a green sitting exactly in the middle looks cool next to red and warm next to blue ("Is green warm or cool?").
4. The effect of wall temperature on thermal comfort was significant, while the color of the goggles had no effect at all; the authors concluded that color could only produce a belief effect ("Does color actually change heat?").
5. One root of the effect is chromatic aberration in the eye, and it varies from person to person depending on the position of the pupil; although most people see red as advancing, some see the reverse or no depth at all ("Depth in space: advancing and receding").
6. All skin tones cluster in the wheel's orange slice; "cool undertone" describes the end of that region closer to red and pink, "warm undertone" the end closer to yellow. The words are relative, not absolute ("Example — "warm undertone" and "cool undertone"").

Sources

• Ou, L.-C., Luo, M. R., Woodcock, A., and Wright, A., "A study of colour emotion and colour preference. Part I: Colour emotions for single colours," Color Research and Application 29: 232–240, 2004.
• Bennett, C. A., and Rey, P., "What's so hot about red?," Human Factors 14(2), 1972.
• Thompson, P., May, K., and Stone, R., "Chromostereopsis: a multicomponent depth effect?," Displays 14(4): 227–234, 1993.

Next step

DT-06 looks at the colors left outside this lesson's spectrum: whether black, white, and gray count as colors, physically and perceptually, the difference between warm and cool grays, and blacks that can look surprisingly colorful. BI-07, meanwhile, unpacks this lesson's Kelvin paradox with its physics: it covers blackbody radiation, the color temperatures of incandescent, fluorescent, LED, and daylight sources, and why low kelvin values look warm.