Although misleading to use the terms ‘light’ and ‘colour’ interchangeably, colour temperature (also referred to as White Balance or WB) is a measure of how cold or warm the light, and consequently the colour, is in a photo.
The temperature of light is measured in degrees of Kelvin, from:
As the temperature of the light changes so too does the colour cast in the photo. However, the time of day is not the only factor in determining temperature, ambient light will play a key role, e.g. cloud, shade or sunlight, artificial lighting etc.
The affect of light temperature on photography is quite simple.
A photo taken will record the temperature of the light as a hue across the overall colour balance and can often take on unpleasant cast from poor lighting conditions, e.g. incandescent bulbs and candles give a very warm light, producing a strong orange cast across a photo that can often lead to unflattering and unnatural skin tones.
The photos below of an autumn landscape show how temperature that is too warm (orange) or too cold (blue) can create unnatural finishes.
Colour temperature is controlled by choosing a setting with your camera menu (WB option) specific to the environment in which you are taking the photograph, such as Tungsten, Cloud, Shade etc. Alternatively the WB setting can be left at Auto to allow the camera to assess the composition for you, although, as with all auto settings on your camera, this can often lead to disappointing results.
Controlling colour temperature with camera film is more difficult and requires a greater understanding of the affect of temperature and a stronger ability to assess a composition.
Camera is usually rated for a specific colour temperature, e.g.
Where the actual temperature is different to the film rating the image can be controlled by the use of filters, e.g. cooling and warming filters, that will compensate for the difference in temperature.
Further control of temperature can be achieved using packages such as Lightroom (particularly where your files are RAW) and Photoshop, both of which will allow you to tone down the overall colour cast in your images.
Ideally however colour temperature should be controlled at the point of taking the photo as this will teach you to assess a composition correctly. Relying on post processing to correct faults usually means the photo was taken incorrectly to begin with.