White Balance & Colour Temperature

White Balance exists to ensure the colours in an image are as accurate as possible (1). It provides an adequate reference point used by the camera to correct any harsh colour casts in an image that occur as a result of the dominant light in the shooting situation. For instance, a white wall lit by a halogen light may result in a yellow colour cast, whereas another type will result in a different cast. This is because- unlike the human eye- the camera lacks the intelligence to automatically adjust itself to different colour temperatures.

Colour temperature is a means of quantifying different types of light (and the light radiated from objects shot under that light). It is measured using the Kelvin (K) scale, which attempts to assign different colour temperatures to a range of common scenes and light sources. Below is a a representation of the Kelvin scale.

The Kelvin scale is used in digital photography (film is different 'kettle of fish' entirely) as a guide to the warmth or coolness a scene. This guide is inseparable from White Balance.  Notice how the lower the number, the warmer the colour.

Our cameras come with White Balance settings used to reflect some of the common lighting scenarios we may find ourselves in (e.g. Direct Sunlight, Tungsten, Shady, Cloudy and Auto). Each setting will give completely different (and very interesting) results. As an example, below are two basic images I shot nearly two years ago using different White Balance settings:

Tungsten/Incandescent:
Choosing this for a scene counterbalances the warmth of the light inside the indoor market, thus rendering the scene very cool.

Cloudy:
I don't know why I chose to shoot this as cloudy here, but in retrospect it  works as a nice comparison. Choosing  this setting assumes there is a cool light present from an overcast sky, thus warming the scene. 

Shooting images in RAW means we can change the image's white balance afterwards by opening it in Adobe Camera Raw, affording us more control over the overall result. For this reason, many people suggest choosing the Auto White Balance (AWB) setting in-camera, then either choosing one of the white balance presets or using the scale provided in ACR. Below is an image shot in RAW using AWB, followed by the same image processed with different white balance settings in ACR:

Auto White Balance (5200k):
Note how Adobe Camera Raw details the temperature as 5200k when the AWB is used in-camera.

2500k:
This is the same image after reducing the temperature to 2500k in ACR. This should give a decent idea of how cooler such a change makes.

Shade (7500k):
And this is after choosing the Shade present in ACR. Note 'shade' is given a value of 7500k on the Kelvin Scale. On the whole, it looks like the Auto setting has it pretty exact in this case.

It is also possible to set the White Balance manually on my camera (Nikon D3100). Unfortunately, this doesn't seem to be working on my camera at the moment, but I will endeavour to update this post as soon as it is back up and running.

1)  http://digital-photography-school.com/introduction-to-white-balance