As many as 30 stores around the state are expected to start selling the drug for recreational purposes from 1 January, dubbed Green Wednesday.
Colorado, along with Washington state, voted to legalise the use and possession of cannabis for people over the age of 21 in November 2012.
Washington is not expected to allow the sale of it until later in 2014.
Store owners had stocked up, prepared celebrations and hired extra security in preparation for their opening on Green Wednesday.
It’s almost the worst of both worlds”
Kevin Sabet Smart Approaches to Marijuana
Under the new law, cannabis will be sold like alcohol. Residents will be able to buy up to one ounce, while those from out of the state can purchase up to a quarter of an ounce.
Cannabis can only be smoked on private premises, with the permission of the owners.
The sale of the drug will be taxed in the same way as alcohol, and state officials have said they expected it to raise millions – the first $40m of which will be used for school construction, The Denver Post reports.
A total of 136 stores have been given licenses to sell marijuana. Most of the shops are based in Denver. Some communities elsewhere in Colorado have exercised their right not to have the stores.
Supporters of legalising cannabis have praised Colorado’s move.
Rachel Gillette, of the Colorado branch of the National Organization for the Reform of Marijuana Laws, said the state “has found an exit strategy for the failed drug war and I hope other states will follow our lead”.
But critics say it sends the wrong message to the nation’s youth and fear it will lead to serious public health and social problems.
“There will still need to be a black market to serve people who are ineligible to buy on a legal market, especially kids,” said Kevin Sabet of Smart Approaches to Marijuana. “It’s almost the worst of both worlds.”