I believe very strongly in final goodbyes. When my Mother passed away we all gathered at my Dad's house after the funeral. While we were all sitting down stairs we saw three deers jump over the fence and into the back yard. One deer came up to the patio doors and stood there for the longest time just watching us. The other two stayed out in the yard. We all sat there quietly and looked at it. Then it just turned around and walked off. It caught up with the other two and all three jumped back over the fence and ran into the woods.
That's all they did.
So I believe my Mother was giving us a final goodbye. Just as I believe that rainbow was Tim Russert's final goodbye.
I'm a rather spiritual person so I believe that the soul of a person, who was loved deeply in life, comes back long enough to soothe the pain of those who are grieving so that the healing process can begin with a sense of peace.
So from Tim Russert it was a rainbow goodbye.