As many as 30 stores around the state are expected to start selling marijuana for recreational purposes from Jan. 1, dubbed Green Wednesday.
Colorado, along with Washington state, voted in November 2012 to legalize the use and possession of cannabis for people over the age of 21.
Washington is not expected to allow the sale of it until later in 2014.
Colorado and Washington are among 20 states to have approved marijuana use for medical purposes. The drug is still illegal under federal law.
Store owners had stocked up, prepared celebrations and hired extra security in preparation for their openings on Green Wednesday.
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.
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 legalizing 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.”