Heat a metal body and describe the colour changes is a familiar question in school science laboratories. As the temperature increases the colour of the metal changes from dull red to bright red to orange and as more heat is applied to yellow to white through to blue - white.
At low temperatures the metal glows red and has a 'warm' subjective appearance that corresponds to longer wavelengths in the emission spectrum, at much higher temperatures the metal glows blue - white and has a 'cold' subjective appearance that corresponds to shorter wavelengths in the emission spectrum.
 
The notion of a blackbody, an ideal (theoretical) thermal source that exchanges energy (absorbs and emits radiation) over a continuous distribution of wavelengths
is used to describe scientifically the colour appearance of a light source.  
In thermal equilibrium, the shape of the broadband spectrum (the dependence of emissive power with wavelength) is determined by temperature alone. Using the concept of quantised electromagnetic oscillations that provided the foundation of quantum theory, this is the celebrated Planck radiation law. Indeed, all bodies
radiate energy at temperatures above absolute zero (−273.15 °C), where molecular motion ceases (quantum mechanical 'zero point motion' remains).

Our laboratory experiment has demonstrated a meaningful fact, as the temperature increases the colour changes, the peak emission is displaced toward shorter wavelengths (the blue end of the visible spectrum and beyond). This behaviour is described mathematically by the Wien displacement law, that relates the wavelength λ_max at maximum power to the absolute temperature T.                

The units of wavelength and absolute temperature given here are micrometre (μm)
and kelvin (K). Emission spectra (the curves are (skewed) bell - shaped) for an ideal thermal source at different temperatures and the Wien displacement law are illustrated, (notice the inverse relationship between wavelength and temperature).
                     

At the higher temperature 10000 K, the radiant power maximum occurs in the ultraviolet band at a wavelength of about 0.29 μm, at the lower temperature 100 K, the radiant power maximum occurs in the infrared band at a wavelength of about 29.0 μm. For terrestrial objects at a nominal room temperature of 300 K (27 °C),
the radiant power maximum occurs at a wavelength of about 9.7 μm. 
The sun can be considered an almost ideal thermal source (blackbody) at 5800 K,
the temperature of the photosphere, the visible 'surface'. This is an average temperature, the local temperature fluctuates. In one second, thermonuclear fusion in the core transforms several million tons of matter into electromagnetic radiation. Approximately 40% of the solar power is distributed in the visible spectrum
(the human visual system is sensitive to stimuli within the wavelength range
0.4 - 0.7 μm), that extends from violet through blue, green, yellow and orange to red, part of the electromagnetic spectrum between the ultraviolet and infrared bands.

For the photographer, assessing the spectral quality of light used to create an image is a crucial element to ensure the correct colour balance.
Think of colour temperature as a single - value characterisation of spectral quality.
Colour Temperature and Correlated Colour Temperature are measured in kelvin,
the unit of absolute (thermodynamic) temperature (after Lord Kelvin, 1824 - 1907), where Kelvin temperature (K) = Celsius temperature (°C) + 273.15.
In terms of the attributes, hue (dominant wavelength) and saturation (variance), colour, as evidenced by the human visual system can be described using x-y chromaticity coordinates on the CIE chromaticity diagram. Shown in black is the Planckian locus, the contour formed by the set of chromaticity coordinates that maps the colour of a blackbody as the temperature changes.
Chromaticity coordinates are derived by weighting the spectral power distribution, using the CIE Standard Observer colour matching functions.                                          

Note that, dependent on the generation process, light can be categorised as
thermal or non - thermal.

Thermal
For artificial and natural light sources, tungsten, sun, ..., that approximate an ideal thermal source, the spectral power distribution provides an unambiguous description of colour temperature (more intuitive than CIE colour space).
The spectral power distribution, the radiant (emissive) power at each wavelength, objectively characterises a light source.
The Colour Temperature of a light source is the Kelvin temperature of
a blackbody that has the same (or similar) spectral power distribution throughout the visible spectrum and therefore emits light of the same
(or similar) colour appearance.
On the CIE chromaticity diagram, Colour Temperature (kelvin) is determined by comparing the chromaticity coordinates of a light source with the chromaticity coordinates of a blackbody (chromaticity coordinates match on the Planckian locus).

Non - thermal
For artificial and natural light sources, fluorescent, aurora, ..., that do not approximate an ideal thermal source, the spectral power is not distributed as a broadband but as a narrowband or as discrete lines, each light source is different.
To characterise spectrally selective light sources, correlated colour temperature is the most practicable spectral quality metric, based solely on the perceived colour.
Given that light sources with disparate spectral power distributions can produce the same perceived colour (have the same chromaticity coordinates) at the same luminance and under the same viewing conditions.
On the CIE chromaticity diagram, Correlated Colour Temperature (kelvin) is determined by comparing the chromaticity coordinates of a light source with the chromaticity coordinates of a blackbody (chromaticity coordinates that represent the closest (metameric) match to the Planckian locus). Shown in blue, (crossing the Planckian locus) are lines of constant correlated colour temperature.
 
For an ideal (theoretical) thermal source, colour temperature and correlated colour temperature are the same.

To summarise, 'warm' tones are associated with light that is rich in red and has a low colour temperature, 'cool' tones are associated with light that is rich in blue and has a high colour temperature.
 
In film and digital photography, reflected (or transmitted) light is used to image
a scene. The natural colour of terrestrial objects is due to variations in the spectral reflectance (light that is not reflected is absorbed or transmitted, the chlorophyll in
a leaf reflects green light but absorbs red light and blue light) of surface features
that are neither ideal diffuse reflectors or ideal specular reflectors, proportions vary. There are many light scattering mechanisms that contribute to our observation and understanding of the world around us. The natural colour of very few terrestrial objects is due to emitted light.
Though combinations of the additive primary colours Red, Green and Blue can reproduce any colour, the perceived colour (hue) of surface features is dependent on the photopic (light - adapted) response of the human visual system and the viewing conditions. A film or digital camera (and photographer's eye) record the product of spectral power (of the light source) and spectral reflectance (of surface features) at each spatial position. A characteristic of the human visual system is chromatic adaptation. The mechanism compensates for the spectral differences between light sources and thus provides an almost colour constant descriptor.
Hence, colour perception (that involves neural processes) is generally independent of the spectral power distribution of the light source (the colour appearance of the leaf
is 'more or less' the same at sunrise, noon and sunset).
Achieving accurate colour reproduction under artificial and natural light that exhibit disparate spectral power distributions remains one of the most challenging problems, fundamental to photography.
   

ARTIFICIAL LIGHT SOURCES
The principal sources of illumination are discharge lamps and incandescent lamps.
A discharge lamp can operate at various pressures to produce a line spectrum or a line spectrum that is superimposed on a (continuous) broadband spectrum. Familiar examples are the fluorescent 'daylight' and 'warm - white' lamps, metal - halide lamps and xenon lamps. The 'daylight lamp' has a range of colour temperatures from 3000 - 6500 K, the 'warm - white' lamp has a colour temperature of about 3000 K. Dependent on the fluorescent coating - vapour combination, the lamp can be tuned to different emission spectra, of late, colour balanced fluorescent lamps that produce uniform lighting have become the standard. Mercury vapour lamps that emit 
blue - green coloured light and sodium vapour lamps (the familiar low pressure sodium lamps used for street lighting) that emit yellow - orange coloured light have discrete line spectra. Vapour lamps that are operated at high pressures and temperatures provide superior colour rendition due to (collisional) broadening of the
spectral lines. Typically, binary or ternary amalgams are introduced. An electronic flash unit operates by means of an electrical discharge through a rare gas (xenon), the line spectrum that is superimposed on a (continuous) broadband spectrum has
a colour temperature of between 5500 and 6500 K, and resembles natural daylight.
An incandescent lamp has a (continuous) broadband spectrum, the spectral power distribution is similar to an ideal thermal source. Common types are the
tungsten - filament lamp (domestic lighting), the tungsten - halogen lamp and the photoflood and photographic lamps. The 100W tungsten - filament lamp has a colour temperature of about 2865 K, the tungsten - halogen lamp has a range of colour temperatures from 2700 - 3400 K and the photoflood and photographic lamps have colour temperatures of about 3400 K and 3200 K. The age and working voltage of a lamp and reflector type (mirrored, ...) may effect the spectral quality of the light and the range of colour temperatures that are available.
Less familiar is solid state lighting that is based on innovative light emitting diode (LED) technology. Lamps produce a broadband (white LEDs are colour temperature tunable) or a narrowband emission spectrum.
Most artificial light sources can produce visually displeasing colour casts, fluorescent a greenish tinge, incandescent a yellowish - orange tinge and an electronic flashtube a bluish tinge.


NATURAL LIGHT SOURCES
The principal source of illumination is daylight (direct sunlight and diffuse light from the clouds and sky). However, the selective absorption and scattering of sunlight by atmospheric constituents (aerosols, gases and pollutants) modifies the solar spectrum. The colour temperature of daylight changes throughout the day (and year) due to the angular position of the sun and from day to day due to the prevailing atmospheric conditions. The colour temperature of average quality sunlight (at noon) is about 5400 K and of photographic daylight is about 5500 K.
The blue colour of skylight is due to selective (Rayleigh) scattering that is inversely proportional to the fourth power of the wavelength, atmospheric gas molecules and particulates scatter blue light (shorter wavelengths) about 4 times more intensely than red light (longer wavelengths). The colour temperature of skylight can range from 12000 - 20000 K. At lower sun angles the direct sunlight has to traverse more of the earth's atmosphere and is depleted of blue light so that colours are biased toward red. Near to sunrise and sunset, the colour temperature of sunlight can range from 2000 - 3000 K. Particulates suspended in the atmosphere (dust, smoke, ...) scatter light selectively to create spectral changes that can colour the sky many shades of red and orange. Clouds filter both sunlight and skylight, the white colour of clouds (and fog) is due to nonselective (Mie) scattering, the larger water droplets
and ice crystals scatter all wavelengths evenly. However, the appearance of clouds
can vary from white to black, due to multiple scattering and shadowing.
The duration of twilight is dependent on latitude (at the equator the twilight period
is brief) and the time of year. Once the sun is below the horizon, the evening sky changes from yellow - orange to dark saturated blue, the morning sky changes are reversed. After sunset, the afterglow (due to light scattering by stratospheric particulates) and the diffused light from middle and high level cloud reflections are the source of an extended range of colour temperatures, at (deep) twilight the
colour temperature may exceed 15000 K.
Most natural light sources can produce visually displeasing colour casts, the lower colour temperature at sunrise and sunset a reddish - orange tinge, the higher colour temperature on an overcast day a bluish tinge.

The principal source of illumination at night is moonlight (sunlight reflected from
the moon's surface), the illuminance at sea level is governed by the elevation above the horizon, the phase of the moon, the earth - moon distance throughout the lunar cycle and the diffuse reflectance properties of the moon. The average visual albedo
(the measure of reflectance) is 0.12. Due to the wavelength dependence of the reflectance, the spectral power distribution is shifted to longer wavelengths (moonlight is slightly redder (warmer) than direct sunlight). Moonlight has a colour temperature of about 4100 K. The ratio of scene illuminance under daylight and
full moonlight conditions is about 150000:1.
On a clear, moonless night, airglow (photochemical luminescence) and direct and scattered starlight are the principal sources of illumination. Though obtrusive
artificial light is a modern world problem, far removed from populated areas, the night sky is not pitch black.

Generally, subjects at sea level receive direct sunlight, skylight (scattered sunlight), cloud scattered light and ground reflected light. In regions of shade the colour balance changes. Generally, subjects at sea level receive skylight, cloud scattered light and ground reflected light. Remember, the colour temperature of natural light is influenced by many environmental factors and varies throughout the day.

The relative proportions of different colours (violet (0.40 μm) to red (0.70 μm)) that contribute to skylight, noon sunlight and 40W tungsten filament (domestic) light are illustrated.
                  

Near to urban centres the juxtaposition of artificial and natural light sources gives rise to a kaleidoscope of colours and intensities. As darkness falls, the blend of artificial and natural light can create a backcloth studded with vivid contrasts
(see London Skyline). To accurately record scenes that are illuminated by light from diverse sources (artificial - natural) with widely different colour temperatures can be
particularly challenging.
Colour temperature meters based on the Kelvin temperature scale use two (red and blue) or three (red, green and blue) filters to characterise the spectral quality of light sources. Compared to spectrocolorimetric or spectroradiometric measurements under controlled conditions, the accuracy of handheld colour temperature meters is limited (especially for non - thermal sources).
 

Under changing illumination, Film Cameras use colour balanced film and optical filtration to manage colour rendition. So that colours appear natural both indoors and outdoors, tungsten balanced (A/B for light sources of 3400/3200 K) and daylight balanced (5500 K) films have colour sensitivities that are matched to the particular conditions. Needless to say, film can be employed creatively, use a tungsten balanced film to record a (noon) outdoor scene produces a bluish cast, use a daylight balanced film to record an indoor scene (under incandescent light) produces a yellowish - orange cast. Optical filters that effectively 'shape' the spectral power distribution of a light source are also used, amber filters decrease the colour temperature, blue filters increase the colour temperature. Light balancing and colour conversion filters (used with film that has an incorrect colour balance, for example, tungsten film used for outdoor photography and with mixed lighting) change the colour temperature of a light source by small and large amounts. The disadvantages of optical filters are: the degree of attenuation, flare and vignetting and for SLR cameras, colouration of the viewfinder image. The colour temperature can also be changed using colour correction gels in front of a light source. A full CTB gel converts tungsten light to daylight, a full CTO gel converts daylight to tungsten light. 

Under changing illumination, Digital Cameras use white balance (WB) technology to manage colour rendition. To compensate for various lighting conditions a fast algorithm estimates the colour temperature of the light source and adjusts the RGB pixel responses to emulate the colour perception of the human visual system,
optical filtration is unnecessary. On the CIE chromaticity diagram, the above process repositions the white point on the Planckian locus. For spectrally selective sources that do not closely approximate a blackbody thermal source, the white point is displaced from the Planckian locus. By setting the green - magenta correction any residual colour cast is removed (bear in mind, the human visual system is most sensitive to yellow - green light). Using the RAW file format, the colour temperature and green - magenta correction can be changed during post processing.
Of course, filters (neutral density, polariser, ...) are useful for special effects and to enhance the contrast - saturation of an image.

So that colours appear natural both indoors and outdoors, the automatic white balance (AWB) introduces a blue shift to compensate for a red colour cast and introduces a red shift to compensate for a blue colour cast. AWB algorithms are classified as Global (grey - world assumption) or Local (white - world assumption), based on analyses of all or selected pixels. In most cases the AWB reproduces
colours convincingly, however, for consistency and quality, select the white balance
preset that most closely matches the lighting conditions and for further refinement
use white balance bracketing. Experiment with the white balance presets, Cloudy, Shade, Direct Sunlight, Fluorescent, Incandescent and Flash (names vary)
and observe the colour changes. Fluorescent lighting can be particularly variable,
some high - end cameras offer alternative fluorescent settings to provide flexibility.
The effect of the Cloudy and Fluorescent settings is demonstrated below.
Notice the global colour shift. The white balance presets are selected using a button
and dial, or multi - selector, typically an icon is displayed on the LCD panel.
For exact colour rendition, use a diffuse white (18% grey card (the card must fill the frame)) or neutral reference that reflects red, green and blue uniformly under all lighting conditions, and perform a manual white balance.

Exercise your creative flair, the camera white balance controls can be used to intensify colours and accentuate a mood. To give emphasis to a cold scene, set the WB to correct for a low colour temperature or select the incandescent preset, the camera response is to add blue. To give emphasis to a warm scene, set the WB to correct for a high colour temperature or select the cloudy/shade preset, the camera response is to add red. Alternatively, perform a manual white balance using a coloured reference (to add cool tones use a red card, to add warm tones use a blue card). The balance of recorded colours is a critical element of your image, 'cool' and 'warm' tones convey a visual message that can subdue or stimulate our emotions.

Be aware of constantly changing lighting, mixed lighting (combined indoor and outdoor lighting) and scenes with predominantly one colour. For optimal colour reproduction, the AWB averages the sampled regions of the scene to remove colour imbalances, however, differences are sometimes over - corrected and introduce localised colour casts. Some cameras feature 'intelligent' AWB that uses scene data and recognition technology. In demanding situations, use the manual white balance setting to achieve greater colour accuracy. At all times, use your knowledge of
colour temperature to obtain the most realistic rendition of artificially and naturally illuminated scenes.

Many parameters can be changed post capture using proprietary or
third - party  image - editing software. For images saved to a RAW file format (shooting parameters (contrast, saturation, sharpening, white balance, ...) are unprocessed), unlimited colour correction (and creation) is feasible,
for images saved to a JPEG file format (shooting parameters (contrast, saturation, sharpening, white balance, ...) are processed), limited colour correction (and creation) is feasible. As a rule, try to achieve the desired effect in - camera (this is more rewarding and can save you time - consuming post processing). Of course, film users can convert images to a digital format for further manipulation.    

One final point, though (Correlated) Colour Temperature is used to describe scientifically the subjective colour or visual appearance of a light source, there are inherent weaknesses. A high colour temperature is perceived as 'cold' and a low colour temperature is perceived as 'warm'. This association is contrary to our instinct, further, the relationship between colour temperature and colour is non linear, our visual perception (colour discrimination) of similar differences in colour temperature is not identical, the colour change at low colour temperatures is greater than the colour change at high colour temperatures, for example, the colour change between 2000 - 2500 K is greater than the colour change between 5000 - 5500 K.
The mired scale (an acronym for MIcro REciprocal Degree) is a measure that avoids the negative aspects of the Kelvin scale. The mired scale is linear, equal increments on the scale correspond (practically) to equal visual increments.

For a colour temperature T (K), the mired value is given by   

Colour film is manufactured to be daylight balanced for 5500 K (MV 182), type A tungsten balanced for 3400 K (MV 294) or type B tungsten balanced for 3200 K, (MV 312). Blue filters (Wratten #80A, ...) increase the colour temperature,
a decrease in mired value. Amber filters (Wratten #85, ...) decrease the colour temperature, an increase in mired value. Furthermore, the mired shift value (MSV) of combined filters is additive, to shoot type B tungsten film in daylight, stacking 81A (MSV +18) and 85 (MSV +112) filters gives a mired shift value of +130.   
Decrease the colour temperature from 5500 to 3200 K with a mired shift of +130,
increase the colour temperature from 3200 to 5500 K with a mired shift of −130. 
For digital cameras that offer fine tuning, the white balance presets can be adjusted, typically at 10 mired intervals.