Any motor vehicle owned by a Colorado resident cannot be parked or driven on a public street or highway unless it is properly registered in Colorado and displays Colorado license plates.
A person who violates any provision of this section commits a class B traffic infraction. A class B traffic infraction has a $15 minimum penalty and a $100 maximum penalty.
If you own a motor vehicle, you must register the vehicle in the county in which you live. Furthermore, the signed registration card must be carried in the vehicle or in the possession of the operator whenever the vehicle is being operated.
Within sixty days after purchase, every owner of a motor vehicle, trailer, semi-trailer, or vehicle that is primarily designed to be operated or drawn upon any highway of this state or any owner of a trailer coach or of mobile machinery whether or not it is operated on the highways, shall register such vehicle with the Colorado Department of Revenue. A person who violates this commits a class B traffic infraction.
If you are a new resident to CO and fail to register all vehicles you own within 30 days of becoming a resident your driving privilege may be suspended, revoked or cancelled. If you had registration and let it expire there may be late fees, fines, etc.
To find out for certain what the penalty for being caught for driving without registration would be in your situation, contact the Colorado Department of Revenue.