This exercise really made me wonder about the default "cloudy" (top picture) where I've habitually had the camera set. The middle one is auto and the bottom is set for full sun even though I was in the shaded woods. I also discovered that auto white balance often works quite well indoors (though not necessarily outside). I think I will be trying out a grey card soon.
@lsanderson55 it's a neutral color that you can take a picture of in the given lighting situation to set a custom white balance with your camera. I understand that you can use plain white paper too, but I've have mixed results with that. My theory is that the white paper maybe picks up reflections?
