I agree with Steve on tartan flashes. They aren't traditional in any sense and are not something that most folks would consider as an upgrade to regular flashes (except the retailers who want to sell them to you as special items).

But with that said, it appears that a lot of kilt makers will throw in a set of tartan flashes with the kilt whether you ask for them or not. I have several sets of them that came with my kilts, and they never get worn. It's an easy thing for kilt makers to make them with some of the scrap tartan. If you're really set on having tartan flashes, it's a good idea to have them made from the same cloth so they match.