As of 8:12pm mountain time, the Colorado secretary of state site shows Amendment 64 leading 671,027 to 610,202, over 52% in favor. The site says that Colorado has 3,647.082 registered voters, with 31.41% of them having had votes counted so far. Assuming Colorado doesn't get 100% voter turnout, does this mean half the vote counted so far? Very promising.
Update (8:24pm mountain time): MPP says exit polling is at 57%.