The color of light is measured by it’s color temperature. Outdoors, ambient light changes color temperature throughout the day. It is warmer (yellower) before sunset and just after sunrise. In the open shade of mid-day it is cooler (bluish).
Our eyes compensate pretty quickly for different color temperatures. A piece of white paper will appear white to us when viewed in either warm or cool lighting conditions. However, the sensor in our cameras, left on its own, is not so lucky. A picture of a piece of white paper taken in open shade will render quite bluish; while another picture of the same piece of white paper, taken near sunset, will be rendered yellowish.
White Balance (WB) is the tool that we use to tell the camera what the color temperature is of the light hitting our subject. If we have warm light hitting our subject, that piece of white paper will be rendered as white if we set the camera’s white balance to warm light. So if the camera’s WB setting matches the color temperature of the light hitting the subject; white, and all other colors, will be rendered correctly. However; if the white balance setting does not match the color temperature of the light hitting our subject, our photo will have distinct a color cast.
If we take a picture in mid-day sun light with the camera’s white balance set to Tungsten the resulting image will have a blue color cast. That is because the tungsten WB is telling the camera that yellow light is falling on our subject. And the camera will compensate by making the image more blue to override the yellow in the ambient light.
Likewise, we can emulate the use of a warming filter with film by using a white balance setting of cloudy. The cloudy WB tell the camera that blue light is hitting the subject so the camera adds yellow to compensate. The resulting image will have a warm color cast.
The top image was taken with the WB set to Auto and renders the color temperature pretty close to the way it was on this winter afternoon just before the sun set. The bottom picture is the same image taken with the WB set to tungsten. Note the distinctive blue color cast the image has. Even though these two images were made at the same time and the camera was not moved between exposures, the emotional content of these two images are quite different.
In summary, we photographers can use white balance to accurately represent colors or we can use it to cause our images to be rendered with a color cast. So which is correct? We’ll discuss that next time.
For those interested, here is the shooting metadata. It is the same for both images except that, as explained above, the top images was shot with WB set to Auto and the lower image was shot with the WB set Tungsten:
Fuji S2 w/ 12mm
1/20sec @ f/11, ISO 100
Matrix, Aperture Priority, Comp +1/2