Since the color samples are taken from the digitization of the original book, with little or no change, I'd bet on wear and introduction of dust over the years

Since the color samples are taken from the digitization of the original book, with little or no change, I'd bet on wear and introduction of dust over the years as the source for most impurities. See https://archive.org/stream/gri_c00033125012743312#page/n45 https://archive.org/stream/gri_c00033125012743312#page/n45