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Home » Challenge #18 Warm and cool colours (Part 1)

Challenge #18 Warm and cool colours (Part 1)

Can we really talk about warm or cool colours? Why not acidic or sweet? Is it a convention or a measurable reality? Can any colour be qualified as warm or cool? And what does this have to do with the colour temperature of a light source?

Let’s say we agree on a definition. Does warm or cool colour imply a difference in perception? Symbolic charge? When it comes to composing a colour palette, are there any recommendations for a warm/cool balance?

There are so many questions on the agenda for this challenge! The difficulty lies not so much in tackling them as in resolving the contradictions in the answers, which vary from one source to another.

As usual, first a little scientific exploration. Gradually heated metal takes on a light-colour that depends directly on its temperature. Of course, heating a body is just one method of generating coloured light, but there is a precise law linking temperature and the colour of the light emitted. And… surprise, the convention used on our taps has nothing to do with this physical reality!

We then take a short diversions into photography, where colour temperature is a parameter that you can control, in shooting or editing mode.

These two aspects already provide a lot of material, even if they focus essentially on light colour. So I’ve decided to split up this challenge.

My best wishes for 2024: a tree reflection transformed by a cool filter in Snapseed.
Snapseed’s vintage #6 filter cools the photograph chosen to send you my best wishes for 2024 (CC-BY-SA 2.5 V. Lacroix).

Heat and light

When heated, the iron emits a yellow light, turning to white as the iron becomes warmer.
Wright of Derby, Joseph, “An Iron Forge” (source: Tate)

When iron is heated, it changes colour. As the temperature rises, it turns red, then yellow, then a dazzling white with a slight blue tinge.

Craftsmen who fire enamelled jewellery, ceramics or even bread know when to put their pieces in the kiln: all they have to do is wait for the “right colour” to bathe the kiln! Depending on its temperature, the colour inside the kiln will change like that of white-hot iron.

The radiation emitted by this iron or kiln can be likened to the “black body radiation”. In fact, all hot bodies emit radiation. Even us! But at body temperature, the radiation is just not visible; we can nevertheless feel it because it manifests itself in the form of heat. That’s how you can detect a fever by putting your hand on a sick person’s forehead. If the temperature rises, the radiation becomes “visible”. Hence the difference in vocabulary: light is radiation that can be detected with the eyes.

Max Planck (1858-1947) determined its content: it is a “soup” of electromagnetic waves of different wavelengths. It can be represented by its spectrum, a tool already mentioned in this article. The spectrum gives the energy contribution of each ray as a function of wavelength. The spectrum of a black body covers three areas: ultraviolet (UV), visible (V) and infrared (IR).

Incandescent light bulbs work on this principle. The heated filament emits light… but also infrared waves, so they are discredited because the energy is “lost” in heat radiation.

Cats love to sleep under incandescent lighting: the light source gives off heat.
Incandescent light, wasted energy? Not for everyone! (CC-BY-SA 2.5 V. Lacroix and Midjourney).

A bath of waves

In fact, we are constantly immersed in a spectrum of electromagnetic waves, only part of which is visible to us. We interpret this part as colours coming from the objects around us.

To illustrate this point , I like to refer to the colourist painter Pierre Bonnard (1867-1947) and in particular to his painting “The large bath”.

Illustration of the wave bath in which we are daily, based on
First variation on Pierre Bonnard’s “The large bath. The multitude of colours evokes visible waves, while the black and white icons remind us of all the others (CC-BY-SA 2.5 V. Lacroix after Bonnard).
Our body is emitting in the Infrared spectrum. And we can feel these waves with our skin, with our hands and illustrated here.
A second variation on “The large bath”, evoking our bodies radiating in the infrared, a radiation that we cannot see but that we can feel (CC-BY-SA 2.5 V. Lacroix after Bonnard).

The spectrum of a black body

The animation below shows the colour and normalised spectrum of radiation from a black body as a function of temperature.

The author simulated the colour of the black body by the mixture of three sources: blue, green and red just like a computer screen does. Their combination –– i.e. addition –– at the intersection of three disks, is identical to the colour perceived. We should rather talk about the resulting hue because the radiation can be more or less intense, and so in a more or less clear version.

Colors and spectrum of the blackbody in function of temperature
Normalised spectrum of a blackbody as a function of temperature © Benjamin Bartlett (courtesy of the author)

Each wavelength of visible light is associated with a colour. Imagine radiation that contains only waves of a given wavelength, for example 580 nm. It is then said to be monochromatic. Well, if intense enough , this radiation would appear yellow-orange. Monochromatic radiation at 480 nm? It will looks blue. 500 nm? Green. And so on for any visible wave perceived in the rainbow.

Furthermore, waves below 380 nm (in the UV range) or above 780 nm (IR) are associated with black to show that they cannot be seen.

Most of the light we encounter every day is a mixture of different wavelengths. Even traffic light is not monochromatic. That said, without a measuring instrument, it’s impossible to know.

The wavelength/colour association is therefore only valid for monochromatic waves, in other words, almost never… with a few exceptions such as lasers, for example.

Also, without an instrument, it is impossible to deduce the spectrum from the perceived colour. In fact, radiation without any waves at 580 nm can appear orange-yellow: mix waves at 530 nm and 610 nm and you have a perfect orange-yellow!

Seeing spectra

Of course, we have a few opportunities to observe monochromatic rays… but juxtaposed. The rainbow, for example, reveals the composition of sunlight.

Albert Bierstadt « After the Shower » (source).

And there are several affordable instruments that allow us to discover the spectrum of any light source. The optical prism is the simplest of them all. Admittedly, it is not a graph giving the energy for each wavelength, but an image. In this image, the brighter a coloured band, the more energy it contains.

Diffraction gratings such as CDs, switched-off screens (smartphones, tablets, computers, TVs) or diffraction glasses multiply the spectrum of the light source before our eyes. But be careful: a spectrum can be superimposed on its own copy, creating new coloured bands. The rays are then no longer juxtaposed: they add up to produce a colour that does not necessarily correspond to a monochromatic wave.

Monochromatic rays can be juxtaposed, either by dispersion or diffraction of the light from the source being analysed.
Tools for viewing spectra. From left to right: prism, CD, diffraction grating, diffraction glasses (sources: CD and grating)

In their light installations, visual artists play with these effects, much to our delight. Take a look below, for example at «Your watercolour machine» by Olafur Eliasson (2009) or his more recent work «Rainbow incubator» in Quatar (2023).

Colour temperature

Surprising as it may seem, the sun – like the stars – is a black body! It emits radiation that is almost identical to that of a black body at 5780°K.

The colour temperature of the black body varies from 1000°K to 12000°K. These are the same colours as those shown in Benjamin Bartlett’s animation (source: wikimedia commons).

The colours of the progressively heated black body form a curve in this diagram, known as the Planck locus, Sometimes these colours are represented in a chromaticity diagram, which allows them to be placed in a wider colour context.

The colours of the progressively heated black body form a curve in this diagram, known as the Planck locus,
Chromaticity diagram of the “Planck locus”, in UV space. The colour of a black body that is progressively heated goes through all the colours of the curve in the middle of the graph: first red (on the right, at 1000°K), then orange (2000°K), then yellow (3000°K) etc, until the white-blue colour (at 10000°K) at the extreme left. Some colour temperatures have specific references: D50, D55, D65. They refer to “daylight” and to standardised lighting colours used in photography, printing and industry. They also refer to the colour of light perceived at different times of the day. When the colour is not produced by a black body but has the appearance of one, it is referred to as a correlated colour temperature (source wiki commons).

That’s what we call colour temperature! A relationship that links the perceived colour of blackbody radiation to its temperature. Note that blackbody radiation is never perceived as green, violet or pink. What’s more, the higher the temperature, the more its colour will evolve towards white-blue. This is the opposite of the convention found on our hot and cold water taps.

The colour of light

Below, a freeze frame of the animation for the temperature of 5500°K, the approximate temperature of the sun. It looks more like the moon than the sun! But that’s forgetting that, as it passes through the atmosphere, the ‘soup’ that is the sun’s rays is profoundly modified, and fortunately so: a large proportion of the UV rays that are harmful to living organisms are blocked by the atmosphere. However, outside the atmosphere, the sun would appear white.

Perceived colour and spectrum of a black body at 5500°K, corresponding approximately to solar radiation before it enters the atmosphere (extract from the animation by © Benjamin Bartlett).

Parts of the solar spectrum

Instead of looking at the whole solar spectrum, we could look at just part of it, as suggested by Bruce Mac Evoy in Color experience Chapter 12.. For example, imagine taking a small window from the short wavelengths. We would note the coloured impression and each time we would add a small window of additional rays, until we reached 565 nm. We would thus report a gradation of blues: darker, then lighter and finally white! If we proceeded in the same way, starting with long waves, we would see a gradation of reds, oranges, yellows and then white too.

Top figure: visualisation of the colour perceived by looking at only the left-hand side of the solar spectrum, or the right-hand side. First by observing a small window, then larger and larger (until reaching 565 nm). Bottom figure: the corresponding colour gradations. The window on the left (shortwave) offers a gradient of dark blue, then lighter, then white. The right-hand window (longwave) shows a gradient of reds, oranges, yellows and white (adapted from Bruce Mac Evoy, Color experience Chapter 12.)

This way of separating the solar spectrum into two parts, left and right, offers another objective suggestion of the ‘warmth’ of coloured light. The shades obtained by progressively increasing the size of the window on the left-hand side give cool colours (short waves). Those on the right, warm colours (long waves). When the size of the window is exactly half, whether on the left or right, a neutral white is perceived.

Representing these nuances in a chromaticity diagram would then make it possible to delimit the warm and cool colour zones.

Crossing the atmosphere: blue sky and red sun

So when the sun’s rays pass through the atmosphere, a series of physical phenomena take place. The spectrum is totally altered. In particular, instead of travelling in a straight line, some rays “bump” into molecules in the atmosphere. They are then reflected in all directions. This phenomenon mainly affects rays of short wavelength; other rays are deflected less. This is why the sky is blue and the setting sun is red: the longer the sun’s rays travel, the more of its ‘blue’ rays will have been lost along the way, leaving only the long-wave rays.

The colour of the sky at sunset and the scattering of rays according to their wavelength.
The diagram shows the path of the sun’s rays as they pass through the atmosphere. The longer the path, the more short-wave rays are scattered. These scattered rays give rise to the different colours of the sky at sunset. Only the longer wavelength rays remain in the direction of the sun, while the short ones colour the sky in blue (adapted from wiki commons).

Cloud molecules, on the other hand, are larger than those in the atmosphere. Here, another scattering rule applies. All the rays are scattered equally, regardless of wavelength. So, when they mix again, the whole appears white: daylight regains the same whiteness it had before passing through the atmosphere.

The effect of atmosphere: how painters interpret it

Is a sunset a precious, old-fashioned beauty? Maybe, but what colourful inspiration for artists! Especially as the above explanation is very general and ignores the specific components of the local atmosphere.

Below is a small selection. You’ll find painters already mentioned in other articles, such as Turner and Delacroix (here), Monet (there), Valotton (in the tonic) and Tom Thompson (in challenge #9).

The Scarlet Sunset c.1830-40 Joseph Mallord William Turner 1775-1851 (source)
“Sunset”, Eugène Delacroix, ca. 1849-50 (source).
« Parliament, sunset», Claude Monet,1902 (source).
“Sunset”, Félix Valotton, 1913 (source).
« Burned Over Land », Tom Thompson, 1916 (source).

The quality of the light

The spectrum associated with sunlight will also depend on the direction in which the light is measured; it will even be different at sea or in the mountains, in the morning or in the evening, in Italy or in Ireland.

The Spectra of various “Daylights” (adapted from Bruce MacEvoy curves show the relative contribution in energy of the rays with respect to wavelength (in manometer).

So we understand the notion of ‘quality of light’, so dear to painters, another way of evoking the composition of light as we perceive it, in all its complexity.

Do you see the different ‘qualities of light’ in the paintings of Albert Bierstadt (1830-1902), whose rainbow you have already seen above? Almost 30 years before Monet, he was interested in variations in light… and also on the façade of a church!

Left « Study for sunlight and shadows »; right «Sunlight and shadows », Albert Bierstadt., 1862 (source of the study and of the final work)
View of Glacier Park or Sunset on Peak “, Albert Bierstadt (source)
“Sunset”, Albert Bierstadt (source)
Buffalo Trail, The Impending Storm“, Albert Bierstadt (source)

To compare the colour of daylight at different times of the day or from different light sources, we use the correlated colour temperature. This temperature is defined as the colour perceived when a black body is heated to a certain temperature, even if the source of the radiation being described is not a black body. This is purely a convention. However, it is specified by adding “correlated”.

Colour temperature and photography

The black body model remains fundamental, however, because it perfectly describes the different types of natural lighting we encounter on a daily basis. Our vision adapts to this very varied lighting and automatically corrects any deviations due to lighting that is too orange, as at sunset, or too blue, as at dusk.

This model also provides a point of comparison for appreciating a scene or lighting it with specific intent.

Cameras and digital cameras also estimate ambient lighting. Often, the on-board system makes the assumption that the most neutral colours in the image are actually whites or greys. If the lighting is coloured, the whites in the scene will adopt the coloured nuance of the lighting. All you have to do is cancel out this nuance by adding the complementary colour to the whole image, and you’re done. This is called adjusting the white balance.

The fruit platter on the left is lit in cool light, like that used on overcast days or at dusk. The one on the right is shot in warm light. In both images, the bottom of the dish appears white. However, magnification reveals a bluish cast on the left and a yellowish cast on the right, which our brain-eye unconsciously corrects (CC-BY-SA 2.5 V. Lacroix).

Modifying colour temperature

Basic image processing software can be used to modify the colour temperature of a photograph. Even on a smartphone!

Already on an ordinary image, in this case COLHORA coloured samples, changing the colour temperature produces a different atmosphere. Doesn’t the image on the left seem ‘cooler’ and the one on the right ‘warmer’? Does this confirm the validity of using the adjectives warm/cool to describe light?

Middle: the COLHORA colour cards photographed with the iphone in automatic mode. On the left, using the iphone photo software, the same photo edited with a colour temperature of -100 (the coolest). On the right, the same photo with a colour temperature of 100 (warmest) (CC-BY-SA 2.5 V. Lacroix).

Here’s an idea for comparing the ‘warmth’ of two colours: take a shade in the central image and see what it looks like in warm and cool light. In the following part of the challenge, we’ll be looking more specifically at ‘colour-matter’, that is the material’s colour.

Instagrammers, influencers and trendy brands are abusing these filters to create a colourful identity that’s always the same! As we’ll see in a workshop article, these practices reduce the colour palette of the original image.

So, as well as producing a restricted palette that is often visually pleasing (see challenge #3), filtering has the effect of unifying colours and reducing glaring discrepancies, in the same way that a painter covers his work with a glaze or a lightly coloured juice.

Filmmakers also use this principle to immerse us in a specific time in the story without indicating it explicitly. A consistent colour temperature is enough to immerse us in a specific atmosphere or mood. Some treatments will evoke the past, the future, dreams or reality. The film Inception is an excellent example. The film’s poster is undoubtedly the result of a “cooling” process.

In conclusion

The colour of light and its temperature

Remember that only monochromatic radiation is associated with a colour; in general, light will be a mixture whose composition we cannot deduce without an instrument, whatever colour we perceive.

However, by convention, we use the “correlated colour temperature” to specify the colour of a light. The colour is estimated by comparing it to the colour of a black body heated to a certain temperature. This convention corresponds to the natural conditions of daylight, to which we have become accustomed through years of evolution.

Association between colour, light and temperature expressed in degrees Kelvin. According to this scale, low temperatures are red and high temperatures are blue (source: wikimedia commons).

Furthermore, when the solar spectrum is divided into two parts, one focused on short wavelengths and the other on long wavelengths, each part appears white. And if the observation windows vary in size, starting with a small one and covering half the spectrum, we see a successive gradation from blue to white for the short wavelengths, and from red to white for the long wavelengths. A tempting justification for describing the nuances produced in this way as cool or warm?

Range of light colours by partial visualisation of the solar spectrum. On the left, cool colours, on the right, warm colours (adapted from Bruce Mac Evoy, Color experience).

However, these two physics-based approaches – the black body approach and the solar spectrum splitting approach – do not attribute any temperature to light on the green-magenta axis.

From colour-light to colour-matter

Digital cameras and image-processing software allow us to modify the overall colour temperature of a scene, either to correct the dominant features of the lighting or to create a particular atmosphere.

Modifying the colour temperature of a photograph has also opened up the possibility of creating cool or warm versions of a given colour: simulate ‘cool’ or ‘warm’ lighting and that’s it! The result is relatively cooler or warmer versions of the original sample, transforming the absolute notion of warm and cool used for lighting in a relative one.

Below, the samples in the middle correspond to the original photograph, taken in automatic mode, above to a simulation using ‘cool’ lighting, and below to ‘warm’ lighting.

Photograph of COLHORA colour samples. In the middle, in natural light. Top, the same photograph simulated in cool light (temperature parameter=-100), bottom, in warm light (temperature parameter=100) (CC-BY-SA 2.5 V. Lacroix).

Alternatively, a glaze, or a lightly coloured transparent film, can be used to warm up or cool down the colour of a painting. This can be done locally or globally.

What’s next?

This part of the “Warm Colours, Cool Colours Challenge #18” focused mainly on the colour of light, from the point of view of physics. It did, however, give us a cue for approaching the question of material colours. A quest that will be followed in the next part.

However, while physics has its laws, our perception has others. Do colour/sensation associations and physiological and perceptual effects depend on the warm/cool categorisation established here?

And what do artists think, how are they using these differences?

Answers in the second part of this challenge!

It’s up to you!

To familiarise yourself with these notions of colour temperature, look at magazine images, paintings, photos on Instagram or on your favourite networks. Did the artist want to simulate a scene shot in cool light? Neutral? Warm? To induce what feeling? What perception? What mood?

In the cinema, pay attention to the different filters that produce a particular atmosphere. What quality of light do you see? Is this filter associated with a flashback? A dream? A character’s story?

Then take your photos and paintings. Edit the photos and play with the temperature to recreate the atmosphere you felt when you took the photo, or to create your own personal vision. Apply a coloured juice, a glaze or a lightly tinted transparent film to your paintings and observe how this manipulation unifies -– but also partially extinguishes -– the original. Follow the next workshop article to learn more about this practice.

The photograph shown in the introduction was taken using the Snapseed application. Vintage #6 filtering has cooled the image overall, giving a more personal view of the subject. A vignetting effect adds a nostalgic touch. Haven’t tried Snapseed yet? Read workshop article #1 for your first steps.

Below, one of my mixed-media encaustic paintings. In the background, a photograph of the Royal Park in Brussels. I’ve covered the whole thing with encaustic paint, which creates a semi-transparent veil that gives depth. At the bottom of the image, a mixture of pigments added to the wax turns this veil blue, simulating a cooler light locally.

“Royal Park in Brussels”, mixed-media with encaustic © Vinciane Lacroix.

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