I'm going to disagree with the others in this thread and say that I always prefer solid flashes to tartan.

The reason for this is because the original garters were long woven bands, and not cut from tartan cloth. They would have been a solid color.

See the garters shown at the bottom of this page:
http://www.almostunwearoutable.com/system/index.html

Theses were wrapped under the knee and tied into a knot. The flashes on our modern elastic garters are meant to emulate the ends of the tied knot of the old style garters. For this reason, solid color flashes are actually the more traditional choice.

I also think that a simple solid color adds a bit of flash to the outfit without being over the top. I prefer to keep things simple whenever possible.

It's not that tartan flashes don't look nice - they do - but I still prefer solid colors for both historical reasons and for reasons of style.
Aye,
Matt