The way this works is that the US sends over information to international partners so as to make them aware of the security features in US passports to look for to confirm that the passports they may encounter are legitimate US passports. This stuff usually gets put into a book for border guard types to use and/or into a look-up file for computer use, and sometimes the updates are done even before the passports are being issued in general circulation.

Some of the reference material used by passport control in the Schengen zone had alerted them to the fact that US passports are no longer to have additional page supplements added to them. Well, guess what happens at times nowadays when some of the Schengen passport control personnel encounter my travel party members' US passports with the supplemental pages added into the passport booklets? If they are aware of that change in US policy and notice the supplements -- and some of them do notice, more often women than men -- then they at times may have to go back and look up when that change in policy came into place. [The elimination of the practice of inserting supplemental pages into US passports was done on the basis of "security" too.]

Today when comparing the links, the diplomatic passport cover is definitely darker ordinary passport cover. My eyes must have been especially poor yesterday, so now I'll have to go check my socks from yesterday and today.