The mauve book goes with all the other muave books, the teal one with the other teals, the reds with the reds, the browns with the browns, and the blue DMG goes with all the other blue books. Obviously.You do you, but not really. You need to remember which colour the book is to find it. Doesn't work for daltonians or colour blind people. Another examples, Numenera books are not the same colour. The player book is mauve while the setting/gm book is teal. What do you do then? 3.5 DMG is blue but the PHB and monster are red and brown. Where does the blue DMG go?
Also: I did state 'some people' on purpose: I do not order my books this way myself, but have known people that do. They seem to prefer the way it looks on the bookshelf (as part of the interior) over the ease of finding what you are looking for. I guess it boils down to the 'form over function' debate.






