Just briefly: in my opinion these covers are comparable to "First Day Covers". There are different suppliers with different cachets, and also in different colors. Whereby the colors should not have any specific meaning.
HPO First trip cover were all philatelic, there are no commercial First Trip covers. First trips were held as events and meant to publicized the transition from the RPO over to the new Highway service.
Cachets were done by many different private entities, very few were done by the Post Office. The cachet colors have no specific purpose.
The top cover in the illustration bears the stamp "POSTMASTER" and his signature? Am I correct?
Yes. HPO covers can be found with many different signatures including postmasters, drivers, clerks, etc.
There is a complete catalog of all HPO Trips here https://stampsmarter.org/Learning/HPOView.html