1860 U.S. Federal Census
In 1860 Colorado was not yet a territory. 1860 Census records for the area that is now Colorado will be found in the census for Kansas, Nebraska, New Mexico, and Utah. The library has:
 a card file name index for the Colorado area of the Kansas and Nebraska 1860 Census.
Manuscript Collection # 639 (United State Bureau of the Census) contains a copy of the 1860 census of Arapahoe County, Kansas Territory.
1885 Colorado State Census 
The only Colorado State Census was taken in 1885. The census was conducted on June 1, 1885. All counties then in the state are represented except Fremont and Garfield. Holdings include census reels, an alphabetical abstract (microfilm), and an index for Arapahoe County in two volumes and on CD.
CENSUS INDEXES
Indexes to the U.S. Federal Census for Colorado 1860, 1870, 1880. The census indexes provide the name of the individual, county and township of residence, and the page number on which the census record appears. 
GENERAL INFORMATION ON THE CENSUS
A Federal Census has been taken every ten years since 1790. The censuses taken through 1930 are available to the public. Census records less than 72 years old are restricted to protect the privacy of living persons. The 1860 through 1900 Federal Censuses counted population as of June 1 st. From 1910 on, the Federal Census counted the population as of April 15 th.
Federal Census records are arranged by state and county. Entries on census schedules are by address. Beginning in 1880 the federal census records are additionally divided into small geographical areas called enumeration districts (ED). If no census index is available, and you know the address of an individual in a large city or county, you can use the descriptions of the enumeration districts to locate the individual more quickly in the census.
The 1890 U.S. Census was almost completely destroyed by fire. Only about 6000 names, none from Colorado, survived.
Mortality schedules, which provide information on deaths during the census year, are available for the censuses taken between 1850 and 1885.

A Soundex is available for the 1900 and 1920 U.S. Federal Census at the Denver Public Library, Western History Department and other repositories.  A Soundex has not been done for the 1910 Federal Census for Colorado. The Soundex provides the following information: the name, county, ED and page number of a census entry. 
Information in the federal census and in census indexes may contain errors. Incorrect information may have been given to the census taker. Handwritten entries may be difficult to decipher correctly. In some cases the quality of the microfilm is poor.