You will I am sure look very smart in whatever you are going to wear,you have given it a lot of thought. This might help you to judge a wee bit better about tweed. Consider that you would go to most weddings wearing a suit if not kilted,yes? Your tweed argyll with the waist coat and normal tie is the kilted equivalent of a suit. Black or brown sporran, it matters not. For what it is worth I wear a black sporran with my tweed.Each to their own. Good luck.