Colorado is a big state.
Any particular area you have in mind?
Lisa Foster's Rocky Mtn National Park book is a good place to start for on and off-trail in the park.
Anything by Jen and Gerry Roach is also good:
Summitpost.com also had excellent info on hikes for specific areas (though aimed at climbing, there is some definite overlap)
Ultimately, the best bet is to grab a good map, see what looks good and then do a little more research based on Da Google. More often than not, Summitpost helps here, too.