Appears that you were on the E-470, a privately owned toll road operated by a quasi-government agency given the authority by state law to impose civil penalties on toll violators. Also appears that the tolls are collected by photographing license plates and sending the bill to the owners.
Also appears that the toll road operator has two options, an administrative civil penalty or a lawsuit in court. According to Colorado statute 43-4-506.5 (4) (c) "If an authority establishes an administrative toll enforcement process, no court of a municipality, county, or city and county shall have jurisdiction to hear toll evasion cases arising on a public highway operated by the authority."
The toll road statutes can be found by navigating through this:
LexisNexis® Custom Solution: Colorado Revised Statutes Research Tool
I think that means if you were assessed a civil penalty based on an administrative toll enforcement process, then you cannot be sued for the money.
I haven't found anything to indicate that there is any time limit for collecting the civil penalties and, normally, such a penalty would have no teeth.
Unfortunately, there is now a vehicle registration hold for vehicle owners who are delinquent and you can bet that's going to be forever or until payment is made.
See:
ExpressToll
So, if you never go back to Colorado, I guess you are home free.