Okay, here’s everything you need to know:
http://www.colorado.edu/registrar/students/state-residency/guidelines
And the answer isn’t entirely straightforward. As an 18-year old who was in a legal guardianship (I am assuming the guardianship was court-ordered - if it wasn’t, then it’s a different situation), you are potentially eligible to establish residency in Colorado, since you have no legal residence elsewhere. But to do so, you’d have to be able to prove that you are entirely self-supporting. If your former guardian (or parent) is continuing to support you, than you wouldn’t qualify.
Assuming that you are able to prove that you’re entirely self-supporting, you run up against another hurdle - a state law presumption that students who seek residency are doing so solely for purposes of qualifying for in-state tuition, which, under state law, is not a legitimate purpose. As a self-supporting, independent student, you could argue that you intend to remain in Colorado, but there’s no guarantee they’d believe you. And it’s highly unlikely that any of this would resolved until after your freshman year.
Your best option, in my opinion, if you really want to do this, is to (1) find out if you can defer your enrollment for a year; (2) move to Colorado immediately and get a job; and then (3) apply for residency after you’ve lived there (and been entirely self-supporting) for a year. Before you do this, in addition to requesting to defer your enrollment, you’d also have to ask if it’s even possible to change your status from out-of-state to in-state after you’ve already been accepted as a student.
So, as far as I can tell, it’s possible that you could get in-state residency, but you’d want to confirm this with the university before you try to do it. It’s possible the school has a “residency officer” who handles just this kind of issue. If not, then you’d want to confirm your plan with a senior member of the admissions office.
Just to be clear, this is my opinion based on a fairly quick review of the information posted on the link above. I have no experience with Colorado residency laws, so I may be missing something. When I have time later this evening, I’ll do some more research and see what I can find.