The human eye – and brain – is very adaptable to the changing colour of light. A sheet of white card will appear white whether you are looking at it outdoors in daylight (bluish), or indoors under the illumination from a tungsten light bulb (yellowish).
Photographic media is not able to adapt in this way. It has to be ‘balanced’ to suit the colour temperature of the prevailing light. The sensor in your camera can be balanced for a wide range of colour temperatures at the touch of a button. Settings include Daylight, Cloudy, Tungsten, Fluorescent, Fluorescent H, Flash, Underwater and Custom.
You will notice that there are settings for Daylight and Cloudy. That’s because the colour temperature of the light is different in cloudy conditions to normal daylight.
If all this is a little confusing, you can set Auto White Balance (AWB). This analyses the light reaching the sensor and sets an appropriate value. However if you want to be certain of the white balance you can use custom white balance; take a photo of a white subject (a piece of white paper is suitable although an 18% grey card is ideal). The camera uses the data from this image to set the white balance for future photographs. You need to repeat the procedure each time the lighting changes.