Gonna sound dumb, but how do you WB with a grey card?

http://www.gretagmacbeth.com/index/products/products_color-standards.htm

I do WB with a gray card by including one in a setup photo under the same lighting I will take the photos with. Then, in Photoshop's ACR, I click the gray area with the eyedropper tool, and voila - the WB is set.

You could also do it the way you suggest. The back side of the gray card is white, and that maybe a better choice for setting a pre-WB.

 

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SteveS, just make sure you set your focus to manual before shooting the grey card.

I got a bunch of the QP cards from B&H. Very good for tricky lighting: you put a small QPCard on the subject, has black grey & white, shoot your first shot, and then use that for contrast and color setting in Photoshop in Curves. You then save the setting as an ".acv" profile and then apply it to any further shots of the same lighting.

 

Steven, in studio conditions where the light is going to remain constant (color temp wise, I mean), I place a gray card in the 'scene' and photograph it, using the studio lights, of course. I then proceed with the rest of the shoot. Later I open that first shot in Nikon Capture Editor and set the white balance using the eye dropper technique (under the White Balance tab.) If a correction in exposure is needed I do that also. With that shot still open I open the photo of the subject I'm going to work on and make any exposure corrections necessary. At that point I click on the gray card shot and go to Edit, copy image adjustments... and in the opening dialog box I choose White Balance. Then make the subjects photo active and go to edit paste. I've found that this will gets me in the very close ballpark but not right on the money each time. I've also found that there are differences in gray cards as we all know by now.
One day all this may be behind us, perfect WB every shot all the time.
Yeah... right..
Jarrell