In addition to providing a means for measuring exposure, a gray card provides a convenient reference for white balance, or color balance, allowing the camera to compensate for the illuminant color in a scene. Gray cards can be used for in-camera white balance or post-processing white balance. Many digital cameras have a custom white balance feature.
A photo of the gray card is taken and used to set white balance for a sequence of photos. For post-processing white balance, a photo of the gray card in the scene is taken, and the image processing software uses the data from the pixels in the gray card area of the photo to set the white balance point for the whole image. Many serious photographers or hobbyists consider gray cards an essential part of the digital photography process.
One available.