A TRUE "Gray Card" can be used for 2 purposes: 1/ Assist the exposure setting process - 2/ Assist the WB setting process. The exposure adjustment should be done first then an image of this gray card is used to set the custom WB.
There are a few things you should keep in mind about gray card:
- You CANNOT print a gray card using an inkjet printer. No matter how well calibrated you system is, you CANNOT achieve a 100% neutrality in the printed gray color. The color of the printed card WILL shift over time.
- A commercial gray card is NOT printed. The card itself is 100% gray throughout.
- The reflectance of the surface affects both of the EV and WB settings.
- The angle of the gray card in relation to the camera's sensor plane makes a difference.
For a home made custom WB source, use the photo paper you normally use, lustre surface preferred. Photo paper (unless it's used for BW photo and has specific Warm /Cool tone dyed in the pulp) is made to be as neutral as possible. If you do a WB off this paper, you also cancel the warm /cool shift of the paper in the process. Thus the color when printed on this particular paper, will be very accurate.
One last note, a "gray card" is not the same as a "grayscale card". A grayscale card has a "grayscale" printed on the card for the purpose of setting up the exposure. The grayscale is normally shown as B&W frames representing 10 values of gray going from a 100% black to a 100% white (or 255-level to 0-level of black). You use an "eye dropper" tool and click on the 255, 128, and 0 levels to set the black, neutral, and white point.