As has already been mentioned, tartan themselves are a little more of a modern thing, and plain flashes are considered more traditional. I'm still like them, though and like to have both plain and tartan flashes in my wardrobe, giving me the choice. I wouldn't however, choose to mix tartans in highland dress. As far as I know, this actually used to be done a long time ago with fly plaids, but is now considered by most in modern times to look off.

In this situation, it seems like it would difficult to acquire tartan flashes to match your kilt, though. Plain flashes that match on of your kilts stripes, would be good bet.