Colo. Rev. Stat. § 38-36-121
The summons shall be directed to the defendants and require them to appear and answer the application within twenty days after the service of the summons, exclusive of the day of service. The summons shall be served as is provided for the service of summons in civil actions in the district court in this state, except as otherwise provided in this article. The summons shall be served upon nonresident defendants and upon "all such unknown persons or parties", defendant, by publishing said summons in a newspaper of general circulation printed and published in the county where the application is filed, once in each week for three consecutive weeks, and such service by publication shall be deemed complete at the end of the twenty-first day from and including the first publication. If any named defendant assents in writing to the registration as prayed for, which assent shall be endorsed upon the application or filed therewith and be duly witnessed and acknowledged, then in all such cases no service of summons upon said defendant shall be necessary.
C.R.S. § 38-36-121
For the competency of newspapers and the construction of publication periods, see § 24-70-106; for service of summons, see C.R.C.P. 4(e) to 4(h).