Some predict that the state capital, Denver, known as the "Mile High City", will descend into drug-induced mayhem beneath a fog of fumes. Others believe legalisation will boost the economy by hundreds of millions of dollars and lead to a swathe of US states following suit.
Subject to final approval of his licence Mr Williams's Apple-style store, called Medicine Man, will open at 8am on New Year's Day. It will be the nearest cannabis shop to Denver International Airport and he is expecting to be besieged. Tour companies have already lined up buses to bring the customers.
A team of expert "bud-tenders" will introduce the uninitiated to products like "Ghost Train Haze", "Blue Dream" and "Kool Aid Kush", and there will be myriad cannabis-infused chocolates, jellies and drinks to buy.
The back of the store will be a giant plate-glass wall so customers can see cannabis plants being cultivated in a cavernous, 18,000 sq ft growing room. VIP tours will take people in to see the plants.
"Whenever people think of my business, they think of a basement and lights hanging from chains, and it couldn't be further from the truth," says Mr Williams. "What's happened is that we have industrialised marijuana. This is a business, with technology and investment. When people see my facility they realise it's not scary. It gives people jobs, it's revenue for the city and state."
Mr Williams, an industrial engineer who previously specialised in aviation navigation, proudly wears a white shirt with the Medicine Man logo on it. Come January 1 his customers will be able to buy those, and other merchandise too. His company employs dozens of people, including a former Harvard biotechnology research student. Like other local businessmen he sponsors community events, and a sports team.
In his corner office, in a quiet suburb of Denver, he sits in front of a computer where software programmes measure product yield and the performance of his workers. On the screen are results and targets for employees in the "trimming room," where the buds are separated from the stems of the cannabis plants.
Colorado has long been in the vanguard of the cannabis legalisation movement in America. For more than a decade it has been legal in the state to sell the drug to people with a doctor-certified medical condition such as multiple sclerosis or cancer, which is what Medicine Man currently does.
Denver alone has hundreds of licensed medical dispensaries. The dispensaries are supplied by regulated growing facilities, located in industrial estate warehouses and as legal as potato farms.
Medical sales of cannabis are now legal in 20 US states. But in November last year, voters in Colorado went further, passing a controversial law known as Amendment 64, allowing its sale for recreational use.
Regulators have said it is "really difficult to figure out" what the market rate for the drug will be in Colorado next year. A study by the University of Colorado suggested the average price would be about US$185 ($226) an ounce, with higher prices for superior quality strains. It estimated one person in eight will buy the drug with a projected annual spend per customer of US$653.
One market research firm estimates the newly legitimate industry will be worth almost US$370 million next year, netting the state nearly US$100 million million) in tax revenues. According to the state's Marijuana Enforcement Division, 136 businesses have already applied to open cannabis shops. Mr Williams was the very first.
Denver is now replete with tour companies urging people to "come and get high in Colorado".
Tim Vee recently switched from the hospitality industry to run cannabis bus tours. He said he was booked up until March. Many of his clients have little or no previous experience of cannabis.
He said: "We'll have to show them how to make a joint, how to smoke."
At a shop selling cannabis pipes in the shapes of donuts and giant squid, an assistant said January 1 was going to be the "apotcalypse", before giggling uncontrollably.
Charlie Brown, who chairs the council committee looking into cannabis issues, said, "People have been telling me for years, 'We don't care if you have marijuana in Denver - but at least tax the hell out of it.'
"This is a big deal and we have to be ready for it. We are going to be the test for the whole country."
• On New Year's Day, Colorado will become the first US state to legalise sales of recreational cannabis.
• Buyers will be able to purchase up to an ounce of the drug each.
• The sales will be taxed at a rate of 25 per cent, with much of the revenue earmarked for building new schools.
• For more than a decade it has been legal in the state to sell the drug to people with a doctor-certified medical condition.
