Yes, it is a photography question, but it requests a chemist answer. Overall concern is about the neutral color accuracy of White Balance cards that are white. It can be a purchase and a usage question, about photographic white balance cards.
OK, stated as one question: How is white plastic made to be white, specifically is it automatically and accurately a neutral color tint?
Regarding inexpensive white plastic White Balance Cards.. How are they made to be neutral white? Not asking details of plastic manufacture, but is the color white always naturally and accurately neutral? Does this white neutral color accuracy need control steps? Other than by adding pigment, might it come out a bit tinted, less neutral?
Some white balance cards are light gray color, which requires mixing in pigments, which is a complication then requiring control and checking for accurate neutral color. Other plastic just seems to be white already.
I know there are many plastics, which is not the question. Some are clear color, some have added color. Some are white? Asking about white.
Some plastics (like say PVC pipe) are white (I think). Is it natural? Is it neutral? In general, how is the accuracy of being neutral white (no color cast) controlled? Is color control necessary? If control, what might be acceptable specifications?
The question is, is there a concern about accurate neutral color of inexpensive white White Balance cards for photography? In practice for White Balance, it seems very close. How close is white plastic to known neutral? Is it assured? Any clue appreciated.