Identifying spiders accurately from photographs is difficult. The only way to be sure what you have is to have an expert look at an actual specimen. You could take the spider to your extension office (210 11th Street, Extension Hall, Fairgrounds, Kremmling) or send it to the Plant Diagnostic Clinic (http://plantclinic.agsci.colostate.edu/). That said, I think this is one of the nursery web spiders. See http://www.ext.colostate.edu/pubs/insect/05512.html for information about common Colorado spiders and their management.