As is now well known, Colorado did not hold a preference caucus but instead held a precinct caucus to elect delegates to go to district caucuses and elect some national convention delegates and also some state convention representatives who would then vote on national convention delegates six weeks later.
I've contacted the Colorado State Republican party asking about March 1 turnout figures but as of this posting have not heard back, nor do I expect to, even though the party's twitter feed is quite active.
The reason the turnout will likely never be made public is because the results would play right into accusations of realized voter suppression, not because registered Republicans couldn't vote, but because they didn't know who they were voting for and the delegates standing for election didn't have to tell them.
Fortunately, we can do a little algebra to come up with a good estimate of a turnout. Back on March 4, several weeks before the Colorado process got the national attention it's received in the last few days, The Denver Post ran an article stating:
The mechanics of a caucus, however, makes it difficult to participate. This year's caucus, despite big turnout on the Democratic side, drew only 11 percent of the party's registered voters. About 5 percent of Republicans attended their precinct meetings.The Colorado Secretary of State's office maintains a statewide tally of party members. As of March 1, 2016 there were 948,658 registered Republicans in the state. Five percent of that is 47,433. The 2012 Republican turnout, in contrast, was 66,027. It is thus reasonable to assume that turnout in Colorado's Republican caucus in 2016 was down nearly 30% from 2012 even while Republican turnout in other states saw double- and triple-digit percentage increases from the previous presidential election cycle.
This, of course, was precisely the objective. North Dakota and Colorado are providing the GOPe templates for the future. Ron Paul was a nuisance and Trump is a genuine threat. The party isn't going to let it happen again.