Colorado had always been either a reliably Republican state or a swing state. It was the tipping point state in both 2008 (sort of) and 2012.
In 2016, Clinton won the state by 5 points, and Trump was the first Republican to win election without taking Colorado since 1908.
I've thought it might be because the Clinton campaign logo looked similar to the Colorado flag just except with an H and a C respectively, and because of the latino vote. (but analysts said it could make a difference, and insisted on a latino vote with Nevada, which was won by Clinton by a smaller margin.)
Combination of a growing Latino vote along with educated suburbanites swinging against Trump.
