David Mullen ASC
Moderator
How do we explain or prove that white and grey are neutral in colour?
White or grey are neutral in color by definition. If they had a color bias then they wouldn't technically be white or grey. As for whether a grey or white card is pure enough in grey or white, that's why you buy an expensive 18% grey card instead of a cheap piece of grey art paper, because it's supposed to be neutral. Finding pure white paper is easier. On the other hand, it's just white balancing, it's not that critical that you get it perfect, you can always color-correct further later in post.