Anyone registered to vote can choose a Republican or Democratic ballot. Many of the people on the Democratic ballot (in my area) are running unopposed, so it was a good opportunity to try to get the least crazy people on the November ballot. Also, the republican ballot has 13 propositions that are not on the Democratic one, so that’s your only chance to have a say in those (and IMO they are really really bad!)
Yeah, Colorado allows for an Unaffiliated registration to let you choose which primary to vote in, so that was my strategy as well. Biden doesn’t need my vote in the primary, but Haley is giving Trump conniptions and I want to keep her in the race as long as I can.