#1. "RE: Digital White Balance Cards or 18% Grey Cards" In response to In response to 0 Fri 01-Jun-12 11:46 PM by elec164
The target needs to be spectrally neutral. Problem with some gray cards is that lower cost ones might not be spectrally neutral. That's not a problem when setting exposure, but can cause an errant per-WB setting.
For that matter any shade of gray can be used, as long as it is spectrally neutral.