I have to second McMurdo's suggestion. It doesn't get much more traditional than diced hose, and either red/white or red/black would be perfect with the outfit you describe. It's plenty "formal", and makes for a nice accent to the rest. Don't worry about the fact that it doesn't match the colours in your tartan. It's perfectly OK, and indeed, expected for diced hose to sort of stand on their own, colour-wise.

Personally, if it were me, I'd go with red/black.