That's just a week before ballots will be mailed out to all voters registered to parties with contested races.
If you're already registered with a party and want to switch, you need to make that switch by July 12, too, in order to get the ballot you desire.
But if you're registered as unaffiliated -- as a third of Colorado voters are -- you "have the ability to affiliate with a party up to and on the day of the election," says Alton Dillard, spokesman for the Denver Elections Division. And if you live in Denver, you'll be able to do that at one of the thirteen voting service centers around the city.
If you don't affiliate, you won't get a ballot -- because the only citywide measure that was going to be on the primary, Jeff Peckman's proposal to create an Extraterrestrial Commission, has been pushed to November.
Denver also has a new service this year, which allows you to trace the progress of your mail-in ballot. Find more details on Denver voting rules here. For information on areas outside of Colorado, try the Secretary of State's website.