Colorado’s unemployment rate crossed into uncharted waters in April, dropping to a record-low 2.3 percent, a mark visited only four other times by any state in recent U.S. history. The rate is so low, it is acting like an anchor on the state’s economy, leaving thousands of jobs unfilled and hurting the ability of businesses to meet customer demands, economists warned. “This is saying that Colorado has one of the most inefficient economies in the country.