Once a mining superpower, Colorado is now a major destination for tourism and recreation as well as agriculture. It's also a powerful draw for geologists of all kinds, who gather by the thousands in Denver every three years for the Geological Society of America's national meeting.
I have also prepared a scan of a very large and much more detailed geologic map of Colorado compiled in 1979 by Ogden Tweto of the U.S. Geological Survey, a classic of geologic mapmaking. The paper copy measures about 150 by 200 centimeters and is at 1:500,000 scale. Unfortunately it is so detailed that it's of little use at anything less than full size, in which all place names and formation labels are legible. I've built full-size versions for each quarter of the state as follows:
All four are 2600x2200 pixels. They overlap slightly so that if you were a maniac, you could print out the four pieces and paste them together into the 5100x4250 master scan. You'll also need the explanation sheet (2000x3100 pixels, 1.7 MB) to identify all the formations shown.
More Colorado resources on About.com:
Colorado Scenic Roads
Colorado Geography, State Symbols & Facts
Colorado Rock Climbing
Colorado Resorts & Spas
Colorado National Parks