It depends on where you live. Here in Longmont they do dyno, gas cap, cat, etc... I know places like Winter Park don't do emissions but most front range cities do emissions.
But don't get fooled, there are mobile emission testing vans. If you pass through them so many times and pass they automatically send you your tags. I know the first time it happened my parents were like "Cool, I guess they don't require emissions anymore." Not true.
Colorado has changed quite a few laws regarding emissions & registration, so just go check out your local cities website. For example, they stopped sending you month tags because they figured out people were putting it off for a month so they could register a month later the next year.
That and it now costs 60 bucks to register my 97, as apposed to only 20 bucks last year.
The whole point to owning an older car is they are cheaper to register and insure.