In Colorado, you can bicycle on Interstate 70 in most places. Exceptsions are for a few miles east of Grand Junction (use the bike path around the tunnels), for 17 miles east of Glenwood Springs (use the bike path through Glenwood Canyon), for 21 miles over Vail Pass (great bike path available), for 11 miles under Loveland Pass (you must cycle over the pass on US6), for five miles above Georgetown (use the frontage road), for 10 miles east of Idaho Springs (use US40 up Floyd Hill), and for 25 miles through the Denver metro area.
Bicycling is prohibited on most of Interstate 25 from Denver north to the Wyoming border (except for one 3-mile stretch and one 11-mile stretch). Once you get out of the Denver metro area, bicycles are allowed on I-25 south to the New Mexico border, except for 12 miles through Colorado Spings and for 8 miles through Pueblo.
Bicycles are allowed on Interstate 76 to the Nebraska border once you get out of the Denver metro area.
Even when allowed, however, bicycling on interstates is generally unpleasant. The most common reasons to do it is if you are desperate to avoid hills and afraid of shoulderless roads, or when there is just no other alternative.