Look at the specific language of the statute that Cliff quoted. Note, that it says "designed to travel on". I recently spoke with the rider of a triked Burgman 650, that had the Tow-Pak kit installed. He said that since the Tow-Pak is removable and leaves the bike unmodified after doing so, it is considered a two wheeler and requires a motorcycle license. I don't know if Colorado statutes have similar language but, I could see how it could be interpreted to allow a motorcycle license with such a kit.