Colour Temperature, expressed in ‘Kelvin’, is a measurement unit for the colour of light that is illuminating your subject. The higher the colour temperature, the bluer it is, and the lower the colour temperature, the redder it is.
For example, candlelight, at 1800 degrees Kelvin, emits a reddish light, and will make colours appear more ‘warm’. On the other hand, light emitted from a cloudy sky emits a blueish light, and will make colours appear ‘cool’.
This can cause a problem in photography, because white surfaces will appear orange in low colour temperatures, and blue in higher colour temperatures – even though the human eye will see white in real life. To compensate for this, cameras are equipped with a ‘White Balance’ setting.
See White Balance
for more information.