From the Canon Digital Learning Center:
"As photographers, our goal is to recreate the scene before us using both light and color. As advanced as digital cameras are these days, they don’t yet match the complexity of the human mind so we have to work with a few technical limitations when trying to record a scene as our eye sees it. Snow covered winter landscapes, in particular, can present some very specific challenges when it comes to getting accurate looking colors.
If you’ve ever looked at one of your winter images and thought that things look a little blue, then you’ve discovered one of the challenges of photographing snow: getting the right white balance or color temperature. White balance is a fundamental camera setting that adjusts color rendition to give a neutral appearance, without any obvious overall color tints or shifts. Cameras come with several White Balance presets (Daylight, Tungsten, Flash, etc.), but difficulties can arise when there are mixed light sources all adding their own color cast. In fact, it doesn’t even need to be a direct source either because all reflected light will have a color cast that’s dependent on the color of the object the light just bounced off of. If there are objects in your image (quite likely!) then you’ve got multiple color casts, in some way."
Check out the entire article on the CDLC Blog.