1. PVC is suspect. They can have color and change over time. Wash it!
I posted, just for the sake of those, who don't know it, that pure PVC is white and does not change the color.
I find the White to be 50 kelvins lower than the grays all the time
50 Kelvins is nothing. It is the
very minimum you can adjust. Picking from the supposedly uniform grey card can yield much larger differences, depending on the spot you pick.
White balance doesn't reuire absolute white
It does not
require white, but the nature of the beast is, that it is easier to get/make something, which reflects
all visible light than something, which reflects
a certain, fixed proportion of all visible light. Therefor, whites are more reliable than greys (although, honestly, I don't care if my white card is more perfect than a grey card).
However, my main consideration is the ruggedness of the PVC card. Now I plan to glue a piece in all of my lens caps. Paper would get dirty, would wear off, would get scratched, etc.