Raingear is definitely a useful component in keeping warm in a cold rain/snow. In those situations, I find the extremities (at least as important).

The photo below was taken in April this year. I did a ride from Alamogordo to Lubbock. The first day, I got a motel in Ruidoso and dropped a rental car in Alamogordo. This let me drop a rental car in Alamogordo and cycle without gear back over Apache Summit. It was 52F in Alamogordo starting out, 48F in Ruidoso finishing and in between 30F with ~2" of wet snow on the west side climbing up. I had tights (no rain pants) and was getting a wet spray from cars but hands (with mittens) and feet (with wool socks) were colder overall. I was happy to go down from the pass to gradual warming and within ~5 miles no more snow along the way. A few days later I was riding in temperatures in the mid 80s.
Photo below was leaving Prudhoe Bay. Temperatures just around freezing with snow. This time it was rain pants and booties as well as mittens and wool socks. Again the extremities as much of a limiter. I was somewhat better off, though that evening after stopping had to quickly get into a sleeping bag to get warm again.