do tartan flashes have to match the kilt?

[Tobus]

Yes. I will get some red flashes and be done with it. :-)

Good choice. Although I would take this opportunity to suggest woolen self-tied garter ties rather than the elastic garters with flashes.

99% of the time, I wear plain red garter ties. You just can't go wrong with plain red. For a lot of people, it's tempting to get crazy with flashes, but IMHO there are so many other accessories and accoutrements in a kilt outfit which can make a personal statement, that it's best to stick with traditional red garters/flashes.