Presidential primaries are conducted by the state but Colorado minor parties have been kept out of the process. From Ballot Access News:

On November 8, Colorado voters will vote on Initiative 140, which would create presidential primaries for parties that polled at least 10% of the vote in the last election. Here is the text. Independent voters could vote in either major party’s primary. However, members of other parties would not be permitted to change their party registration on presidential primary date for the purpose of receiving a particular party’s primary ballot. The presidential primary would be in March. The two major parties would set the date within March, assuming they agree.

From Colorado’s Initiative 140:

COLORADO VOTERS EXPERIENCED DISENFRANCHISEMENT AND PROFOUND DISAPPOiNTMENT WITH THE STATE’S SYSTEM FOR PARTICIPATING IN THE PRESIDENTIAL NOMINATION PROCESS IN 2016.
ON THE DEMOCRATIC SIDE, ATTENDANCE AT CAUCUSES OVERWHELMED MANY PARTYRUN EVENTS, WITH LONG LINES OR OTHER PROBLEMS LIMITING MANY ELiGIBLE VOTERS’ ABILITY TO TAKE PART.

THE STATE’S REPUBLICAN PARTY CHOSE NOT TO HOLD OFFICIAL PREFERENCE POLLS, ESSENTIALLY LEAVING ACTIVE REPUBLICAN VOTERS WITHOUT A SAY ON THE PARTY’S POTENTIAL NOMINEES.
AND, GIVEN THE RULES FOR PARTICIPATING IN CAUCUSES, THE STATE’S LARGEST GROUP OF VOTERS –MORE THAN 1 MILLION UNAFFILIATED ELECTORS –WERE DISENFRANCHISED ALTOGETHER