Having said this, the difference between the images taken in sunlight and shade are very visible, the shot in the shade is distinctly bluer in colour caste. Within my workflow software, Lightroom, the as shot white balance is clearly blue on the colour slider, correcting to shade shifts the balance towards yellow.
5D2, 24-105mm, 105mm, f/8, 1/180s, ISO 100
5D2, 24-105mm, 105mm, f/8, 1/20s, ISO 100
In each of the above images my observation at the time of shooting was that Heidi's face was similar in colour tone, just differently lit, clearly my eyes white balance as well or better than a camera (I have experienced this strongly underwater, looking up from a colour screen on the back of a Digital camera the underwater world shifts dramatically towards the Blue and then resets itself in my brain).
20D, 24-105mm, 105mm, f/4, 1/200s, ISO 100
The third image is the problem one, clearly not taken in low sun in a wintery Munich. I looked through my library for any shots of Heidi taken close to sunset and found this one. It is taken close to the equator off the coast of Borneo, about 30 minutes before sunset, the image has very limited shadow indicating that the Sun is very low. The difficulty is that the skin tones are very different due to tanning, however, there is a very clear shift towards red. I will work up a sunset shot of Heidi once the weather improves and add that to this blog entry.
I would certainly correct the shade image as it looks completely washed out, simply adjusting the WB to from Daylight to Shade provides the following:
This is clearly better balanced and the skin tones are much more similar to those of the Sunlit shot (still needs some correction, but illustrates the point).
The sunset shot needs no correction in my eye, it is too orange, but that is an attractive colour and very suggestive of the low tropical sun, especially today where it is -8C outside!.
As I shoot digitally and exclusively shoot in RAW these colour castes are less of an issue for me at the time of shooting. I find that the Auto WB on the latest Canon DSLR's works very well and where it fails I can adjust in Lightroom work flow. My key concern these days is to manage the exposure correctly and more accurately place focus where I want it to be. The only time I am really concerned about WB is when shooting using artificial light. In these cases I carry a grey card and use this to set a custom WB in the camera. coupled with a hand held light meter this has proved to be pretty accurate.