I did a Nov 19-Dec 10 trip in 2010. The biggest problem wasn't the temps, but the lack of sun as stated above. I found the following to be a good resource as it gives sunrise/sunset times by camp by month. Found it to be pretty accurate. Click on the link to 'Boatman's Almanac' here: Camp sun/shade - Rafting Grand Canyon
On 11/24/10, Thanksgiving Eve, we had snow reach the canyon floor. A blizzard had moved through on the rim. It was cold, but not horrible. Not keeping dry socks available on the raft was my mistake and ended up with Chillblains.
In the upper canyon, water froze overnight. But, definitely warmed as we proceeded downstream.
I wore my drysuit for the big rapids, but most of the time it was splash pants and a wool top. I had a hypothermia kit accessible at all times that included an extra sleeping bag, jet boil and tent. Had to use it for a swimmer from a flipped raft.
A friend had purchased a "Mr. Heater" and that was nice in the mornings to take the chill off. Also, filling thermos containers with hot beverages in the morning for use through the day.
The people who suffered the most didn't drink enough water.