I was recently reading a book that had to do with getting color right in the digital age and was surprised to find that gray cards come in different 'colors' of gray. The trusty Kodak gray card I'd had since the early nineties turns out to be neutral in terms of exposure, but was never intended to be used for white balancing a digital exposure.
I did some searching and settled on a new 'digital' gray card and did some experiments. When using the new card to create a custom white balance setting on my camera I found the old card had a definite color cast to it. The photo at left illustrates a photo taken using a (new) neutral gray card.
Digital cameras try to automatically or manually compensate for different 'colors' of light. Sometimes you are photographing something that doesn't really have anything neutral in the photo. To be confident that the exposure is correct you can take a photo of the gray card in the same light and then use that photo as the reference for the white balance. The method differs between cameras, but the important point is that just because something looks white or gray, it isn't necessarily truly neutrual. Our eyes adjust to different colored lights - a white sheet of paper looks white to the eye whether outside on a sunny day, in the shade or indoor with artificial lighting.
I find a gray card particularly useful to properly adjust for the light color when I bounce a flash off a wall, ceiling or reflector panel. I simply take a photo of the gray card in approximately the same position as I will be taking the final photo, and use it as the reference for my camera's custom white balance.