Prince William was wearing the Windsor white tie jacket with red collar and cuffs. At the French state banquet in July he was about the only royal male not wearing it. Googling the subject I could only find William wearing a black tie jacket with the Windsor red collar and cuffs, so perhaps this was his first time with the white tie version, let me know if there was an earlier occasion that I've missed.

I think it had been a while since the last state banquet at Windsor before the one in July this year so perhaps William was 'caught out'. In my opinion he looked smarter with the Windsor jacket, it's a great tradition to continue!