I recently purchased a grey card set from here (comes with a grey card, white card, and a black card):
I took a photo that had both the white card and grey card in the image. Then I opened the photo in photoshop and adjusted the WB using the eye droppers inside the 'Levels' dialog. I created two separate layers - layer 1 had the original image with the grey point selected on the grey card, and layer 2 had the white point selected on the white card.
It is my understanding that both layers should produce the same white balanced image, assuming that the image is not overexposed for the white point (which it was not). But the colors on both images were different! Can someone please explain why this is?