I am so new to this I am reluctant to make a statement but...
I think if it looks good, then that is the reason to wear it. If it looks bad that is a reason not to wear it. I don't believe that just because HRH wears it, it therefore looks good and/or needs to be emulated. The Emperors New Clothes comes to mind. Although the brown sporran and black shoes combination is not horrible, I personally didn't see a photo where matching leather would not have looked better. As literally every photo shoes that combination it is surely a thoughtful choice on his or his valet's part. The fact that he is not shown to wear brown shoes in any of the photos, supports the comment that he wears black shoes as that is the traditional color of shoes worn with highland dress, and therefore non-negotiable to him. I just don't believe we should give someone the power to influence what looks good just because of who they are. It either looks good to us, or it doesn't. I advocate wearing what you think looks good, regardless of who else may be wearing it. But that is probably the Yankee in me.

Thanks to the originators of this thread, and all of the contributors. I have really enjoyed the photos and the comments.