In 2016, Colorado passed Prop 107 which established open presidential primaries and allowed unaffiliated voters to vote in them and Prop 108 which allowed unaffiliated electors to vote in the primary elections of major political parties without declaring an affiliation with the party.
1-7-201. Voting at primary election.
(1) Any registered elector who has declared an affiliation with a political party that is participating in a primary election and who desires to vote for candidates of that party at a primary election shall show identification, as defined in section 1-1-104 (19.5), write his or her name and address on a form available at the voter service and polling center, and give the form to one of the election judges.
(2) If the name is found on the registration list, the election judge having charge of the list shall likewise repeat the elector's name and present the elector with the party ballot of the political party affiliation last recorded.
(2.3) An eligible unaffiliated elector is entitled to vote in the primary election of a major political party without affiliating with that political party. To vote in a political party's primary election without declaring an affiliation with the political party, any eligible unaffiliated elector shall declare to the election judges the name of the political party in whose primary election the elector wishes to vote. Thereupon, the election judges shall deliver the appropriate party ballot to the elector. In addition, any eligible unaffiliated elector may openly declare to the election judges the name of the political party with which the elector wishes to affiliate and complete the necessary forms. An eligible elector must separately date and sign or date and initial a declaration of affiliation with a political party form in such manner that the elector clearly acknowledges that the affiliation has been properly recorded. Thereupon, the election judges shall deliver the appropriate party ballot to the eligible elector.
(3) Forms completed by eligible electors, as provided in subsection (1) of this section, shall be returned with other election materials to the county clerk and recorder. If no challenges have been made, the forms may be destroyed pursuant to section 1-7-802.
(4) Party ballots shall be cast in the same manner as in general elections. An elector shall not vote for more candidates for any office than are to be elected at the general election as indicated on the ballot.
(5) Instead of voting for a candidate whose name is printed on the party ballot, an elector may cast a write-in vote for any eligible candidate who is a member of the major political party and who has filed an affidavit of intent of write-in candidacy pursuant to section 1-4-1101. When no candidate has been designated by an assembly or by petition, a write-in candidate for nomination by any major political party must receive at least the number of votes at any primary election that is required by section 1-4-801 (2) to become designated as a candidate by petition.
(6) The provisions of subsections (1), (2), and (4) of this section shall not apply to a primary election conducted as a mail ballot election pursuant to article 7.5 of this title.
1-4-702. Nominations of candidates for general election by convention.
(1) Notwithstanding any other provision of law, a political party may choose to change from the nomination of candidates by primary election to the nomination of candidates by assembly or convention for all offices including, but not limited to, united states senator, representative in congress, all elective state, district,and county officers, and members of the general assembly if at least three-fourths of the total membership of the party's state central committee votes to use the assembly or convention nomination process; except that nominations by major political parties for candidates for lieutenant governor shall be made by the party's candidate for governor pursuant to section 1-4-502 (3). Such vote of the party central committee shall occur no later than October 1 of the year preceding the year in which an assembly or convention nominating process is to be used.
(2) A political party nominating candidates by party assembly or convention shall nominate the candidates of the party and make such nominations public not later than seventy-five days before the general election.
(3) Whichever method of candidate selection is chosen by a major political party as between primary election, assembly or convention, all of the candidates for that party at any level of office in that election year must be selected by such method, except that the requirements of this provision shall not apply to a primary for president of the united states if such an election is held.