Could this be a different species of Toxomerus? The Ontario Syrphidae and Syrphidae of Oklahoma both include only 3 species (T. marginatus, T. germinatus, and T. politus), but there are a number of additional species of the genus in North America. Colorado could well have additional species that might not be readily separable from the 3 already on this site. Unless somebody knows how to separate these, it might be best to leave this at the generic level.

I added a few more shots of one of these to my own fly gallery today, and two more specimens from a few days ago, one of which kept its wings folded against its body. Would any of these extra shots help to either match or rule out any subspecies here?