or, if you are looking for a particular flower:
I have identified these Colorado wildflowers with the help of many books and a few friends. My most frequently used books are:
Southwest Colorado Wildflowers, Ferns, and Trees also provides an extensive searchable list of flowers, usable throughout the state.
Eastern Colorado Wildflowers lists the flowers by Latin name, common name, and family. This site also has some tips on telling look alike flowers apart.
When my sources disagree about the latin name or family of a wildflower, I go with the folks at http://plants.usda.gov/. This database is very complete, for the entire United States, but not searchable by the appearance of the flower.
If you really want to know about Colorado wildflowers, I suggest you buy one of these, or a similar guide. These folk actually know what they are talking about, and provide a lot more information than I do.