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, Shade, Cloudy, Tungsten, Fluorescent, Flash and Custom.
You will notice that there are three daylight settings. That’s because the colour temperature of the light is different in the shade or in cloudy conditions. Flash also gets its own setting, even though most flashguns are balanced to give the same colour temperature as 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.
Colour temperatures are measured in kelvins, or K (note that there is no degree symbol). Values relevant to photography range from about 1000 K (candlelight) to around 10,000 K (very blue sky). Average daylight is around 5000 to 5500 K; tungsten bulb illumination is around 2700 K; early morning and late evening sunlight is around 3000 to 3500 K.
Some EOS models allow you to balance the sensor to a particular colour temperature value. Check your instruction manual for details.