I don't know what the Colorado State web site says... but from my experience at Chatfield State Park and Golden Gate Canyon State Park, if you make a reservation on Reserve America and don't show up, they do not let your site go as you have already paid for it. It may say they might give it away after 24 hours, but I have NEVER seen it and have seen empty sites for entire weekends... Whether its right or wrong, they've been paid and that's that.
When you show up with no reservation to find a spot, even if the spot looks empty, their board may show a reservation and it's not available. Colorado has become a 6 month out planning State for that kind of camping. I'm sure other states are the same. I remember going to San Elijo State beach in So Cal for a week back in 2005. A local looked at my Colorado license plate and said, how did you get a reservation for here in July... Just like you buddy, 6 months out to the minute.