White Balance. This picture was taken in RAW mode. The camera was set to Auto White Balance. When shooting in Raw the White Balance is not applied to the photo, it just shows how it would look using the current White Balance setting. To get an accurate reading for the current light, we use a Neutral Grey/White card as reference.
In shooting RAW, this can be done before or after the current photo as long as the light has not changed appreciably. The next image shows the Grey card photo, and the current photo with corrected White Balance. It is important to note that this card is different from the 18% grey Kodak card. It was designed for Exposure correction, but it is not color neutral, so it is not recommended as a White Balance reference card.