Digital Outback Photo have published a good article about correcting colour in your images using a gray card, a technique that works equally well for both digital and film cameras.

“In the real world, the lighting conditions are usually less than idea for color correction; outdoor or indoor lighting is seldom close to the photographic ideal color temperatures for ‘daylight’ or ‘tungsten’. This can be resolved fairly easily by capturing a photo of a gray card in the same lighting conditions and correcting to the gray.”

Website: Digital Outback Photo - Color Correction using a Gray Card