I don't have the iPad 6th gen but I just opened iPad Mini 6, flipped from Dark Mode to default and cranked brightness to max and it is WHITE.
There is a "Reduce White Point" setting in Accessibility, Display & Text Size, but I couldn't find anything to make it replicate that yellowish color in your pic.
My bet is on Acorntoy's suggestion. I can only get my 6 towards that look of the Stock app by manipulating an "on" Night shift.