Tartan flashes are expensive (I've heard close to $50) but look very nice, I think. Scottishkilts.net sent me matching tartan flashes with my kilt. Otherwise, Phil and Episcopalscot's advice is good. Experiment and see what ties it all together the best, your opinion will probably be right on. I'm not versed on the formal rules, but I believe pure white is very formal, so off white might just be spot on.