I think I remember that Colorado is one of the worst states when it comes to verifying eligibility to vote. Colorado allows people to show up at the polls, register, and vote without showing any evidence of residence. You can merely state your intention of moving into the voting district and be allowed to vote. Withdrawing your registration would not prevent you from just showing up and voting. (unless I'm getting this all wrong: I didn't look it up, but there were many threads about this in the past)
In Colorado you have to reside in the state for 22 days prior to the election. While we do not have photo id almost everyone uses their drivers’ license as we can just swipe them and check registration and address. Yes you can register and vote on same day.