When you customize the white balance in SLR photography, do you use white cards as medium gray or pure white?

Your first understanding is correct. When customizing white balance, the use principle of white card and gray card is the same. Of course, in most cases, it is easier to define white balance with white cards, because white paper is easy to find, and gray cards are generally used for photography, and the paper that is casually found is not allowed (it may be color cast); Generally speaking, we use more gray cards when measuring light. The white card for white balance is placed in the environment you need to shoot, perpendicular to the camera optical axis (let the surrounding ambient light shine, not in the shadow).

When customizing white balance, the steps are as follows (I use Nikon camera): generally, choose the exposure mode of the camera as A or S, enter the camera menu, select the shooting tab, find the white balance option, click Enter, select manual preset, click Enter, select measurement, click Enter, and rewrite the existing measurement data? Click Yes, then find a blank sheet of paper (copy paper is enough), let the blank sheet of paper fill the picture, focus (if you can't auto focus, switch to manual focus), and shoot, so that the camera can get the custom white balance data, and then shoot the picture you want immediately (note that the white balance mode of the camera is still custom white balance, which needs to be restored to other white balances and set manually), so that the white balance of the shot photo will be shot according to the white balance you defined.

I wonder if I made myself clear. I hope my answer is helpful to you.