Many searches for information in Census data will require a lot of work. But there are some tools and portals that can make finding basic information easier. Below are some important ones.
First, from Census.gov:
A quick way to get population counts (in a plain text file) for counties from 1900 to 1990 is by plugging your state's two-digit postcode abbreviation into this URL, which gives the table for Colorado:
Other institutions have also developed useful tools for finding Census data: