Here's where the Colonel and the Major fall out over "uniform regulations". A tartan waistcoat, cut on the bias (as sported by my kinsman, above) or a waistcoat in a deep red (not bright red) look just fine, and are properly worn with traditional formal Highland attire.

Solid colours, other than a deep red, don't seem to work as well with the Prince Charlie coatee, and prints and patterns really don't work at all as they tend to be far too busy.

So, want something other than basic black? Tartan, deep red, and black silk all make excellent alternatives to the "ordinary" black barathea waistcoat.