Colorado is definitely pretty miserable for wintertime paddling. Gore is by far the best for a IV-IV+ paddler. The main rapids are pretty fun even down to 400 cfs with a creeky class IV feel and deep channels. The entrance to tunnel changed this year, so maybe it won't be as bad at low water, but who knows. It tends to be pretty locked down from mid-Nov to mid-Mar though (depending on how cold the weather is) and I guess that would be winter.
So get really friendly with Shoshone.