Colo. Rev. Stat. § 31-11-108
Any initiative or referendum petition shall be signed only by registered electors who are eligible to vote on the measure. Each registered elector shall sign his or her own signature and shall print his or her name, the address at which he or she resides, including the street number and name, the city or town, the county, and the date of signing. Each registered elector signing a petition shall be encouraged by the circulator of the petition to sign the petition in ink. In the event a registered elector is physically disabled or is illiterate and wishes to sign the petition, the elector shall sign or make his or her mark in the space so provided. Any person, but not a circulator, may assist the disabled or illiterate elector in completing the remaining information required by this section. The person providing assistance shall sign his or her name and address and shall state that such assistance was given to the disabled or illiterate elector.
C.R.S. § 31-11-108