Colorado is a little different as far as emissions are concerned. Only the front range counties (think Colorado springs to Longmont, if not Ft. Collins) have required emissions testing. If you are running a regular plate you have to get tested. New cars get tested every other year but the older ones are every year. Plus they have these new "mobile" test trucks that sit on the sides of a busy road or on ramp and somehow can test the output as you drive by.
To avoid the BS, you can get a classic plate (must be 25 yrs old), but to get the plate you have to still get it tested once. The plate is good for 5yrs though. It must pass the standards set for the year of production. They will do a quick visual too. Depending on where you go they may look at everything or just look under the hood. The last time I took one in (my dad's 78 Malibu) was in Colorado springs and all the guy did was open the hood and look for the PCV, EGR and air pump. Never did crawl underneath to look at the cat. (the kid did give an offer to buy the car though!)
Keep in mind the older vehicles can just go to a local auto shop for the testing, like a Lube shop or other independent. Not having to put up with the testing bs down here I'm not sure if it has changed, but the newer cars used to have to be tested at a state run shop and rolled on the dyno. That might have been replaced by the moble trucks.
So even with the classic plate you will have to be tested. Which means you might need a cat back on there.
