Which is the correct white balance?

Old horse-drawn wagon, Lake Nakamun, Alberta

Before you are two images, each differing by one single factor.  They have both been white-balanced to the snow, but the colours are clearly different.  So what is going on?  Snow is, after all, supposed to be white. 

Although we think of snow as being white, it can actually be many colours.  In places where coal is used extensively the snow takes on grey hues.  A certain species of protozoa is red in colour, and when present in significant numbers dyes the snow its distinctive shade.  Compact snow, converted to ice, may lose the dissolved gasses originally present and take on its familiar blue tones.  Then there is yellow snow - we all know what that means.

The property of snow that we need to understand is that it reflects all wavelengths of visible light.  If you were to place a string of  Christmas lights on the snow at night, the snow around each light would reflect the emitted light displaying the associated colour.  That is what is happening here.  Direct sunlight has a relatively even balance of light making up the spectrum.  The brighter areas of the scene are lit by it.  The shaded areas, on the other hand, are illuminated by the sun indirectly.

Indirect sunlight will bear the cast of wherever the light comes from.  On a sunny day shaded areas receive reflected light from areas immediately around them, but more importantly, the bulk of the light comes from the open sky above.  Blue sky carries with it more blue than other shades.  Just like a blue Christmas light will cause snow around it to turn blue, the light from open sky is also mostly blue.  The result is light in shaded areas on sunny days is predominately blue.

This brings us back to the shots above.  The top image was colour balanced to the snow in shade, which resulted in having the blue colour cast removed.  Look at the wagon; notice how the wood seems a more natural tone.  The bottom image was balanced using the brighter sunlit snow, causing the shaded areas to appear bluish.  Compare the wagons in both shots and you will see that the bottom wagon is also very blue for the same reason.

Here is the general thought then:  Colour balance for the subject.  If the subject is in shade, balance for that light source.  If the subject is in sun, then for that source.  This works well for the most part, although there are exceptions to everything.  I prefer the top photo because the wagon is not maligned with the blue colour cast of shadows.

Thanks for reading.