It's one thing when a state won't allow them to be registered (or even transferred in). I hate it, but I "understand". It's a completely different thing to revoke registration of vehicles that were registered (without fraud). To me this is worse than unfair, it's really taking the value out of something without compensation. I can only imagine the amount of frustration and anger in Colorado over this.
BTW I was told Minnesota tried to do this a few years ago, and lost a court case over it.. but I can find no record of it, so it may have been some sort of settlement vs an actual decision. (In the end MN changed their DMV laws to allow HMMWVs and CUCVs to be registered for on-road use, even though if you check the DMV website it STILL claims otherwise 3 years later!)
My hope is the CO legislature "fixes" the problem they apparently created and is able to instruct the DMV to reverse any Off-Road Only titling.. even if, in the end, they no longer allow HMMWVs to be titled in the state.