A gray card is a piece of cardboard with a uniform neutral gray printed on one side; you can purchase one at your local photography store. The next time you're photographing a portrait, have your subject hold the gray card chest-high. Make an exposure, and then set aside the gray card and continue shooting. You'll only need it again if the lighting conditions change.

Back home in Photoshop, open the photo of the subject holding the gray card. In Levels (Command-L, or Image > Adjustments > Levels), select the center (gray) eyedropper, and then click on the gray card in the image. The job of the dropper is to take the sampled color and equalize the value of the R, G, and B channels, thus making gray. Since we know the gray card should be neutral, clicking on it with the gray dropper will balance it -- and move all the other colors into correct values along with it. Before closing Levels, click Save, then name and save this Levels adjustment. Now when you open the other shots from your protraiture session, choose Load in the Levels dialog and choose the profile that you just made -- it should enab le you to color-balance the entire set of shots.

Of course, the gray card isn't helpful for photographing, say, soaring eagles, but for still-lifes and portraits, it's a valuable tool.

Select the middle (gra) dropper in the Levels dialog, then click the gray card in your photo.