Colorado is funky. The state runs emissions testing for gas vehicles, but diesels have to go to private shops. The roadside testing on the on ramps only works for gassers and it is only for saying you passed and don't have to go in to a station. Never know where they will be and have to pass by them several times in a year for them to exempt you.
There are several large diesel groups in CO and I'm sure someone at one of the shops would help you out, but I don't know for sure. All the guys I've talked to are pretty straight shooters when it comes to the emissions testing stuff because of what could happen if the state caught them cheating. I have a 7.3 so the only thing I have to do is reconnect the CCV for my testing. There are some counties that don't require emissions for diesels, but you would end up to the North or out east of the Denver Metro area.