Sweet Irony is that Cherry Creek is I think the only outdoor public range in the Metro area. So, firearms are definitely allowed. The park rules say that concealed carry is ok, but it doesn't specifically ban open carry. The only reference is to CRS 18, which is the generic criminal code section. IF there is a law in there banning OC in a state park, then OC is banned. It's possible it might be posted at the park, but I don't know since I never go there. I'm checking the CRS to see.
I'm driving out of town on Friday, but I would have loved to have gone.
UPDATE: nothing in CRS 18, Article 12 that deals with firearms prevents or even mentions firearms in state parks. So unless there is a sign at the park specifically banning OC, I can't find anything preventing it.