I do know that homes were at one time available through either Montgomery Ward or Sears, Roebuck.
You chose the exterior design and fit your choice of interior design to the exterior.
Wards or Sears would ship you all of the parts that you needed to build the house. You either hired a contractor to build it, or you built it yourself. I know of at least one in Central City because I worked on it in the 1970's.
There might have been other "Mail Order" homes available through different regional vendors.
Also, there were books published at that time, much like today, where drawings of finished homes were shown and you could order the plans and bills of material. This was such a popular venue for home design that there was a specialty magazine published in Denver that featured the Designs and floor plans of homes built in Denver.
Another option might just be that since Colorado was so new, there may have been a lack of architects and the existing plans were just reused over and over again by different contractors.