Voters in Nebraska approved a similar measure. California, Michigan and Washington approved affirmative action bans in previous years.
In Colorado, the proposal would have amended the state constitution to declare that the state may not discriminate against, or grant preferential treatment to, individuals or groups based on race, color, sex, ethnicity or national origin.
The ballot language didn't specifically mention affirmative action.
Initially, Amendment 46 seemed headed for easy passage. Polls suggested it had support among a wide range of voters.
Opponents, including Gov. Bill Ritter, called the measure deceptive, saying it would have, among other things, jeopardized outreach programs for minority children.
Opponents credited an intense door-to-door campaign among voters for the ban's rejection.
Detractors also included Denver Mayor John Hickenlooper and three NCAA Division I men's basketball coaches.
"We definitely had moments when we though it was a David-and-Goliath fight, when we thought no one was listening. But they were," said Melissa Hart, a University of Colorado law professor who led the fight against Amendment 46.
Ward Connerly, the black businessman and former University of California regent who orchestrated the effort to ban affirmative action in Nebraska and Colorado, blamed its failure here on the size of the state's ballot, the nation's longest with 13 other measures, as well as the groundswell of support for Barack Obama, the nation's first black president-elect.
Colorado, a traditional red state, went for Obama in Tuesday's election.
Faced with so many complicated ballot issues - from a proposal to define when life begins to another that would raise oil severance taxes - Coloradans were in no mood to approve the ban, Connerly said.
"More than anything else, more than anything, it was the tendency to just vote 'no,"' he said.
Connerly said he would still try to pass affirmative action bans in other states that allow citizen initiatives and left open the possibility of trying again in Colorado.
He had originally envisioned a "Super Tuesday for Equal Rights" this year featuring proposed bans in five states but could only get ballot measures in Colorado and Nebraska.
