You're thinking employment verification, or checking that the places you listed on your work history are correct. As an employer, you open yourself up to litigation if you say anything more than dates and titles. I mean there are stories of employers being called about a past, problematic employee, and saying things like "I could get sued if I told you how their performance was, wink wink." But... :-)

I don't believe in Colorado it is illegal to say anything more, it's just not worth it to risk possible litigation.