I know of the White’s Encyclopedia of United States Stamps color guide, and yes it is quite expensive. For a couple of reasons. First, the amount of study time that went into the research of the guide(s) is worth a great deal. White spent a lifetime researching United States philatelic colors, inks, papers, and repairs. Second, there are only a small number of copies produced. So that produces rarity in the publication. The amount of collectors who are wanting advanced color research is small. Advanced collectors spending thousands on rare stamps are willing to pay a decent amount for an advanced color guide like White’s. Thus it so.