Not sure on CO as I haven’t lived there for years. In CA every aspect of the engine has to be stock or CARB Approved for anything 76 or newer.
There are some exceptions. Anything registered as a diesel is exempt up to 1997. There are also three county tier levels most are the strictest tier—must pass visual, sniffer or obdII if so equipped and no CEL may be on or stored.
The middle tier is for somewhat rural zip codes, it’s sniffer or obdII. So for older vehicles, if it can pass the sniffer, you’re good to go.
The best tier, which my zip happens to be in—the vehicle is only required to be smogged on change of ownership, but never after. Just annual registration fees.