Personally, I prefer solid color with tartan. Usually, I pick a single color from the sett and use that. If I am wearing light colored hose (light grey, muted green, or cream hose), I go for one of the darker colors, and vise versa.

I actually make my flashes out of light weight fashion denim, and I think they look rather nice. Of course, for casual or business wear I wear flashes most ould say are way too long, but it's a bit of my style.