USGS Geologic Map viewer and data download
Some Colorado geologic maps authored by the USGS are available for free download via the United States Geological Survey (USGS) National Geologic Map Database (NGMDB) and the national NGMDB MapViewer. [A zoomed-in view of only the State of Colorado on the MapViewer.]
The CGS regularly submits newly-produced geologic maps to the USGS NGMDB via the STATEMAP Program. However there may be a significant delay before they are available. For the most recent maps and publications, check the CGS store for hundreds of free map and GIS data downloads.