Different light sources have different color temperatures. The lower color temperatures are associated with longer wavelengths (redder) while higher color temperatures are associated with shorter wavelength (more blue). Unlike human, camera cannot intelligently decipher the scene it sees and tell what is white and what is not. Therefore, to make sure white is really white, we need to correct the color of the photo to remove the color cast from the light source. This is typically a straight forward procedure if there is a uniform and consistent light illuminating the scene. However it will be challenging if there are mixed lights of different temperatures (colors). This is actually quite common for landscape photography. Not only the light from the Sun has different colors at different time of the day but also the color of light is modified by clouds or the shade. To illustrate the problems, please see the following different versions of the same photo with different white balance settings.
I took the following photo when the Sun was setting. The Sun behind me cast a warm, golden glow on the remote hills and a long shadow from a cliff. The camera, a Nikon D200, was set to Auto WB. This is definitely not a spectacular photo by any means but looks normal to my eyes.
The following is the same photo but with white balance corrected in Nikon Capture NX2 using the snow under the direct sun light. The warm golden color is gone from the snow under the direct sun light. The sky is more bluish. Do you notice how blue the snow under the shade is? This is not strange as the shaded area receives more of its light from the blue sky. Usually this is not noticeable because human eye and brain tends to ignore the color case when looking at the original scene. With a clear reference of true white on your monitor, the color cast becomes very obvious.
The next photo is again the same photo but with white balance set based on the snow in the shade. It appears to be closer to the Auto WB shot.
In the next photo, I first set the white balance of the entire photo using snow under direct sun light as reference but selectively turned the snow in the shade in to neutral gray.
Which one do you prefer? Do you want to see snow as true white or neutral gray no matter where it is? Do you only care about the white balance of snow under direct sun light, or in the shade, or as predominant part of the photo? Or do you prefer to go for the most pleasing editing result ignoring the color cast all together in photos like this?
I played with three different presets for the daylight white balance in Nikon Capture NX2. The following are the results. It seems to me that the Cloudy WB is very close to the Auto WB result the camera recorded for the shot. If you hate making choices, trusting your camera may not be a bad option. 😉