Originally Posted by neuroanatomist
FWIW, the Canon manual suggests using a white card to set white balance. In reality, gray vs. white makes no real difference as long as they are spectrally neutral. The 18% gray card was primarily used for setting exposure, not white balance (because in the days of the gray card and manual exposure settings, white balance didn't exist - we bought color-balanced films for the conditions we'd be shooting in, with no way to change 'on the fly'). However, gray cards are spectrally neutral so they work well for setting a custom WB.
<div style="clear: both;"]</div>