All light sources have their own colour, although the naked eye does not really notice this. If you have ever noticed that the same scene appears different at different times of the day or season, this is due to the fact that it is being illuminated by a light source who's color temperature is changing. This varies from the deep red light of a candle, yelllow light from a tungsten bulb, the golden light from the late evening sun, or the typical blue light at dusk or dawn ( twilight )
Our brain automatically compensates or adjusts for these variations in light. For certain things like the pages of a book, they always look white whether read in daylight or under a household bulb.
With Film we were restricted to one kind of a film, either balanced for daylight or tungsten. Digital cameras handle this process differently and can take care of individual exposure independent of one another. The camera automatically measures the color temperature of the scene and adjusts the “white balance” of the image electronically to give natural-looking colors in all lighting situations. This is a significant advantage over film, where filters often had to be used at the shooting or printing stage to ensure that the picture did not have a strong color cast.
Although the automatic white-balance system does a good job on most occasions, it is not foolproof. Most cameras have a manual override so that you can alter the overall color of the image to get a more accurate representation or a more artistic representation of the same scene. White balance is set manually by choosing the lighting type (cloudy, sunny, and so on), or by specifying the color temperature precisely using the Kelvin (K) scale. On some models, there is a custom white balance setting, the white balance can be set by using the camera to take a color temperature reading from a sheet of white paper.
Some scenes include several light sources—each with a different color temperature. You can only correct for one light source using the camera, so in mixed lighting, choose a white-balance setting that best suits the main subject or balances the main light source.
Sometimes you have one kind of a light source in a room but through the windows you can see the outside, illuminated by a different light source. You have no option but to adjust for one and let the other one go wrong. In controlled situations, one may use filters, add artificial light or even use gels on the windows instead of adding anything in front of the lens