Colorado can be generally divided into (east to west):
The age of the rock formations ranges from Precambrian granites exposed in the Rockies to Quaternary deposits from the last episode of glaciation and stream erosion.
(Fountain Valley Trail; 2011; Greg Willis, photographer.)
Geology is usually specific to a site, so try including geographic keywords (county, quadrangle) or the names of formations in your search. Geology Top Picks lists general geology databases. In addition, for Colorado try:
Colorado Scientific Society -- for local field guides
Field Trip Guidebook, Geological Society of America Centennial Meeting, Denver, Colorado (1988). Holden, Gregory S.
Geology of Colorado and Western Ore Deposits (1893). Lakes, Arthur.
Guide to the geology of the Golden area (1938). Van Tuyl, F. M.; Johnson, J. Harlan; Waldschmidt, W. A.; Boyd, James; Parker, Ben H. Quarterly of the Colorado School of Mines 33(3) p. 5-32.
Introduction to the geology of the Golden area, Colorado (1934). Johnson, J. Harlan. Quarterly of the Colorado School of Mines 29(4) p. 7-36.
Table Mountain lavas and associated igneous rocks near Golden, Colorado (1939). Waldschmidt, W. A. Quarterly of the Colorado School of Mines 34(3) p. 3-62.