I have seen the book of the exhibition of these photographs. You can buy the book for between $10 - $20. and you can find it on alibris.com if you put a search in for the title: "Photographer to the Tsar".
What a beautiful insightful album. Sergei had a labourious job to shoot those. I haven't heard if the U.S.A even used color then, Kodak had the dye transfer process and there were autochromes but I don't believe that was until later on. Princells' book has a camera process of shooting through three filters onto three pieces of film. I have seen the dye transfer process and it is fantastic in quality and archival value. And as pointed out the registration of negs is very critical. The dye transfer process printed three different negs on matrix film that had registration holes for alignment. Again Bravo for this addition..Don