Population totals for Colorado, counties & places, 1870 to 2010 are available here.
Historical Census data can also be obtained through the University of Virginia. This site presents data for the people and the economy of the U.S. for each state and county from 1790 to 1960.
Population estimates are produced annually by the State Demography Office. The most recent estimate available is for the year of 2018 and were finalized in September 2019.
User Lookup: This option allows the user to select any combination of counties or municipalities for the years 2010 to current.
County Profile: The County Profile database allows the user to select county-level population estimates along with other variables, for years 1985 to current.
Population forecasts are produced annually by the State Demography Office.
These tables illustrate the population for future years, based on plausible courses of future population change (see Forecasts Methodology for more information.) The forecasts are for Colorado counties and are available for the years 2000 to 2050.