Using Manual White Balance
If you're using manual white balance to achieve a true white, there are some things to consider. First, there are the more obvious suggestions, such as making sure you fill as much of the lens' field of view with the card to minimize white-balance errors. Avoid placing the card near highly saturated surfaces (like a red tablecloth, for example). Also, be sure the card is in the same light as your subject.
A less obvious clue to successful white balance deals with exposure. When you're using a white card, make sure it isn't overexposed. If the camera's CPU is attempting to adjust the red, green and blue channels, and those channels are oversaturated, the software in the camera won't make the correct adjustment. If you use the typical white card and are viewing it through an LCD screen, consider drawing a line on the card so that you can see whether or not you're overexposing it. A gray card has less of a problem for exposure.
There are times when you don't want perfect white balance. In those cases, you're just looking for white-balance control. Just as portrait photographers might use a warming filter, you can use different-colored cards to adjust the warmth or coolness of your image. Instead of using a neutral card, you might use a light-blue card when you want to warm up (in photography's, not physics', parlance) the picture, or a light-pink card when you want to cool it down. There are cards you can purchase if you don't want to make your own (check out www.warmcards.com/digital_camera.html).
While initially you might consider white balance a challenge in working with digital cameras, in reality, it's an opportunity. Manual white-balance control offers the digital photographer an on-location tool that film photographers never had—an almost continuous color temperature adjustment.