In an often posted photo of the Royal family at a white tie state affair, the Duke of Edinburgh can be seen wearing a 3 button Balmoral tartan vest with (I think) a Stewart tartan kilt. His hose, while not tartan coordinate in color with the Balmoral tartan. Other than a few of the McLeay paintings showing mixed tartans I haven't seen this anywhere else.

Does anyone have photos showing this in practice? Or, failing that, knowledge of this mixing and rules or protocols that apply to this practice?

This is purely to satisfy my own curiosity, I do not plan on mixing tartans myself. Thanks.

Rondo
P.S. I tried searching for the photo but it is late and my search-fu has failed me.