And at a handful of shops, owners were scrambling to plan celebrations, set up coffee stations, arrange food giveaways and hire extra security to prepare for potential crowds and overnight campers ready to buy up to an ounce of legal weed.
While smoking pot has been legal in Colorado for the past year, so-called Green Wednesday represents another historic milestone for the decades-old legalization movement: the unveiling of the nation's first legal pot industry.
"It could be crazy. Or it could be crickets out there. Who knows? No one's ever done this before," said Robin Hackett, manager of BotanaCare in Northglenn, a suburb of Denver, who planned to have a DJ to greet shoppers.
Preparation for the retail market started more than a year ago, soon after Colorado voters in 2012 approved the legal pot industry. Washington state has its own version, which is scheduled to open in mid-2014.
Pot advocates, who had long pushed legalization as an alternative to the lengthy and costly global drug war, had argued it would generate revenue for state coffers and save money in locking up drug offenders.
Still, setting up regulations, taxation and oversight for a drug that's never been regulated before took some time.
Colorado set up an elaborate plant-tracking system to try to keep the drug away from the black market, and regulators set up packaging, labeling and testing requirements, along with potency limits for edible pot.
The U.S. Justice Department outlined an eight-point slate of priorities for pot regulation, requiring states to keep the drug away from minors, criminal cartels, federal property and other states in order to avoid a federal crackdown. Pot is still illegal under federal law.
With the additional police patrols, the airport warnings and various other measures, officials are hoping they had enough safeguards in place to avoid predictions of public health and safety harm from the opening of the pot shops.
But they confessed anxiety about the opening of retail sales.
"We understand that Colorado is under a microscope," Jack Finlaw, lawyer to Gov. John Hickenlooper and overseer of a major task force to chart news pot laws, recently told reporters about the first day.
Would pot-shop parking lots be full of overnight campers and crowds lined up to buy pot? Would sellers run out of marijuana? Would shoppers abide by state law and refrain from using pot publicly, or would clouds of pot smoke drift through neighborhoods?
Interviews with dozens of marijuana critics, legalization advocates and industry workers showed many share Finlaw's opinion.
Colorado's on a big green stage as dozens of state and foreign countries mull changing marijuana laws.
Since Colorado and Washington state's votes, Uruguay became the first nation to regulate pot. Across the U.S., several municipalities, including Portland, Maine, have ratcheted back criminal penalties for possessing small amounts of pot.
Critics fear the changing global marijuana approach is setting up Colorado and other places for serious public health problems.
"This movement in public policy basically conflicts with the essence of bringing greater mental health and public health," said Patrick Kennedy, a former Rhode Island congressman and chairman of Smart Approaches to Marijuana, which opposes legalization.
Marijuana supporters, meanwhile, were hoping that they'll make the best use of their chance to show that legalization can work.
Maura Foss, compliance manager at Breckenridge Cannabis Club in the ski resort town, is upping inventory from a normal 5 or 6 pounds to 50 pounds of weed for Green Wednesday.
Foss said she wasn't sure how long the pot would last given a steady stream of tourists calling and coming in seeking to buy it.
"We've been scrambling to get the basics ready, and we're as ready as we can be," Foss said.
Kristen Wyatt can be reached at http://www.twitter.com/APkristenwyatt