I think it really depends on the wedding. If it's your run of the mill event, meaning only the groomsmen are wearing anything like a "Tuxedo", I think wearing a knit tie would be a nice way to differentiate yourself from the crowd of poorly dressed men.
However I must agree that black is not advisable for a wedding.
If it's a more formal event then I would probably shy away from the knit tie idea and go for something in silk.
I wore a white spread collar FC shirt with a icy lilac colored silk tie (suit had narrow-ish lapels and the tie was about 2.75") and purple and white PS to a wedding. It was smashing success. I was the only guy there with a PS and received many compliments. Scored massive bonus points with the date, she loved it as she was wearing a purple-ish colored dress.